11g memory

Editor's Notes

• #2: Memory Memory refers to the way we record events, information, and skills. Encoding: Acquiring information from the world and storing it in memory Levels of processing (Craik and Lockhart): Two types of encoding strategies Shallow: Encoding for surface features (less successful) Deep: Encoding for structural relationships and meaning (more successful) Storage: Holding on to information for later use Retrieval: Getting information back when it is needed. Two ways to retrieve information: Recall: Supplying information in response to a cue or question Recognition: Deciding whether information was encountered before   Three Memory Systems Sensory memory: Where information first enters memory system. Large capacity, short duration (split seconds); some of it is encoded or stored in the STM. Short-term memory (STM) or working memory: Where we use and are aware of memories Small capacity (7 plus or minus 2 items) Chunking: STM capacity increases by recoding information into larger, meaningful units Short duration (30 seconds) Rehearsal: STM duration increases through repetition of information Long-term memory (LTM): Relatively permanent store of information, unlimited capacity, and long duration. One can transfer memories from STM to LTM by rehearsal. Implicit: Memory for skills and motor patterns Explicit: Memory for facts, events, and meanings Semantic: Memory for general meanings and information Episodic: Memory of specific personal events
• #3: As early as 1861 Pierre Paul Broca had discovered that damage to the posterior portion of the left frontal lobe (Broca's area) produces a specific deficit in language (Chapter 1). Soon thereafter it became clear that other mental functions, such as perception and voluntary movement, can be related to the operation of discrete neural circuits in the brain. The successes of efforts to localize brain functions led to the question: Are there also discrete systems in the brain concerned with memory? If so, are all memory processes located in one region, or are they distributed throughout the brain? In contrast to the prevalent view about the localized operation of other cognitive functions, many students of learning doubted that memory functions could be localized. In fact, until the middle of the twentieth century many psychologists doubted that memory was a discrete function, independent of perception, language, or movement. One reason for the persistent doubt is that memory storage does indeed involve many different regions of the brain. We now appreciate, however, that these regions are not equally important. There are several fundamentally different types of memory storage, and certain regions of the brain are much more important for some types of storage than for others.
• #4: MOTOR AND SENSORY CENTERS take up less than one-half of the cerebral cortex in humans. The rest of the cortex is occupied by the association areas, which coordinate events arising in the motor and sensory centers. Three association areas—the prefrontal, parietal temporal occipital, and limbic—are involved in cognitive behavior planning, thinking, feeling, perception, speech, learning, memory, emotion, and skilled movements. Most of the early evidence relating cognitive functions to the association areas came from clinical studies of brain-damaged patients. For example, the study of language in patients with aphasia has yielded important information about how human mental processes are distributed in the two hemispheres of the brain. More recently, studies using experimental animals, including genetically modified mice, have provided important insight into the neural mechanisms that underlie mental functions other than language. Genetic manipulation in experimental animals also helps evaluate the relative contribution of genes and learning to specific types of behavior. Even the highest cognitive abilities have a genetic component. Composing music is an excellent example. Music conforms to complex, unusually abstract rules that must be learned, yet clearly it has genetic components intertwined with its learned aspects. The great composer Johann Sebastian Bach had many children, five of whom were distinguished musicians and composers. His only grandson also was a composer and harpsichordist to the Court of Prussia. In 1730 Bach proudly wrote that he was able to “put on a vocal and instrumental concert with my own family.” Today's neural science is cognitive neural science, a merger of neurophysiology, anatomy, developmental biology, cell and molecular biology, and cognitive psychology. This discipline is grounded in the idea, first stated by Hippocrates more than two thousand years ago, that the proper study of the mind begins with the brain. Until two decades ago the study of higher mental function was approached in two complementary ways: through psychological observation and through invasive experimental physiology. In the first part of the twentieth century, to avoid untestable concepts and hypotheses, psychology became rigidly concerned with behaviors defined strictly in terms of observable stimuli and responses. Orthodox behaviorists thought it unproductive to deal with consciousness, feeling, attention, or even motivation. By concentrating only on observable motor actions behaviorists asked, What can an organism do and how does it do it? Indeed, careful quantitative analysis of stimuli and responses has contributed greatly to our understanding of the acquisition and use of “implicit” knowledge. However, humans and other higher animals also have “explicit” knowledge. Thus we also need to ask, What does the animal know about the world and how does it come to know it? How is that knowledge represented in the brain? And how does explicit knowledge differ from implicit knowledge? The modern effort to understand the neural mechanisms of higher mental functions began at the end of the eighteenth century when Franz Joseph Gall, a German neuroanatomist, proposed that particular mental functions are discretely localized in the brain. By the mid nineteenth century clinical neurologists, who regarded their patients as “natural experiments” in brain function, studied brain lesions at autopsy to discover where particular brain functions are located. In 1861 Pierre Paul Broca, using evidence from the damaged brains of aphasic patients, convinced the scientific establishment that speech is controlled by a specific area of the left frontal lobe. Soon afterward the control of voluntary movement was localized and the various primary sensory cortices for vision, audition, somatic sensation, and taste were delineated. Neural science is only beginning to analyze the nature of the internal representations that cognitive psychologists have insisted intervenes between stimulus and response, and the very real dynamic mental processes studied by psychoanalysts. Neural science has not yet been able to address directly the subjective sense of individuality, will, and purpose that is common to us all. In the past, ascribing a particular behavioral feature to mental process that could not be directly observed meant that the process must be excluded from study because no reliable techniques were available to examine brain function in the context of behavior. In this part of the book we show that, because the nervous system has become more accessible to behavioral experiments, internal representations of experience can be explored in a controlled manner. A key concern of cognitive psychology and psychoanalysis is the relative importance of genetic and learned factors in forming a mental representation of the world. These disciplines can be strengthened by the insights into behavior that neurobiology now offers. The task for the years ahead is to produce a psychology that—though still concerned with problems of mental representation, cognitive dynamics, and subjective states of mind—is grounded firmly in empirical neural science. Summary Human memory entails a number of biological strategies and anatomical substrates that are unlikely to be explained in terms of any particular cellular or molecular mechanism. Primary among these are a system for memories that can be expressed by means of language and can be made available to the conscious mind (declarative memory), and a separate system that concerns skills and associations that are essentially prelinguistic, operating at a largely unconscious level (procedural memory). Based on evidence from amnesic patients and knowledge about normal patterns of neural connections in the humanbrain, the hippocampus and associated midline diencephalic and medial temporal lobe structures are critically important in laying down new declarative memories, although not in storing them (a process that occurs primarily in the association cortices). In contrast, procedural memories for motor and other unconscious skills depends on the integrity of the premotor cortex, basal ganglia, and cerebellum, and is not affected by lesions that impair the declarative memory system. The common denominator of these categories of stored information is generally thought to be alterations in the strength and number of the synaptic connections in the cerebral cortices that mediate associations between stimuli and the behavioral responses to them.
• #5: Figure 27.5. Cortical mapping of the language areas in the left cerebral cortex during neurosurgery. (A) Location of the classical language areas. (B) Evidence for the variability of language representation among individuals. This diagram summarizes data from 117 patients whose language areas were electrically mapped at the time of surgery. The number in each circle indicates the percentage of the patients who showed interference with language in response to stimulation at that site. Note also that many of the sites that elicited interference fall outside the classic language areas. (B after Ojemann et al., 1989.) The first person to obtain evidence that memory processes might be localized to specific regions of the human brain was the neurosurgeon Wilder Penfield. Penfield was a student of Charles Sherrington, the pioneering English neurophysiologist who, at the turn of the century, mapped the motor representation of anesthetized monkeys by systematically probing the cerebral cortex with electrodes and recording the activity of motor nerves. By the 1940s Penfield had begun to apply similar methods of electrical stimulation to map the motor, sensory, and language functions in the cerebral cortex of patients undergoing brain surgery for the relief of focal epilepsy. Since the brain itself does not have pain receptors, brain surgery is painless and can be carried out under local anesthesia in patients that are fully awake. Thus, patients undergoing brain surgery are able to describe what they experience in response to electrical stimuli applied to different cortical areas. On hearing about these experiments, Sherrington, who had always worked with monkeys and cats, told Penfield, “It must be great fun to put a question to the [experimental] preparation and have it answered!” Penfield explored the cortical surface in more than a thousand patients. On rare occasions he found that electrical stimulation of the temporal lobes produced what he called an experiential response —a coherent recollection of an earlier experience. These studies were provocative, but they did not convince the scientific community that the temporal lobe is critical for memory because all of the patients Penfield studied had epileptic seizure foci in the temporal lobe, and the sites most effective in eliciting experiential responses were near those foci. Thus the responses might have been the result of localized seizure activity. Furthermore, the responses occurred in only 8% of all attempts at stimulating the temporal lobes.
• #6: Figure 62-1 The medial temporal lobe and memory storage. A. The longitudinal extent of the temporal lobe lesion in the patient known as H.M. in a ventral view of the brain. B. Cross sections showing the estimated extent of surgical removal of areas of the brain in the patient H.M. Surgery was a bilateral, single-stage procedure. The right side is shown here intact to illustrate the structures that were removed. (Modified from Milner 1966.) C. Magnetic resonance image (MRI) scan of a parasagittal section from the left side of H.M.'s brain. The calibration bar on the right side of the panel has 1 cm increments. The resected portion of the anterior temporal lobes is indicated with an asterisk. The remaining portion of the intraventricular portion of the hippocampal formation is indicated with an open arrow. Approximately 2 cm of preserved hippocampal formation is visible bilaterally. Note also the substantial cerebellar degeneration obvious as enlarged folial spaces. (From Corkin et al. 1997.) Memory Can Be Classified as Implicit or Explicit on the Basis of How Information Is Stored and Recalled More convincing evidence that the temporal lobes are important in memory emerged in the mid 1950s from the study of patients who had undergone bilateral removal of the hippocampus and neighboring regions in the temporal lobe as treatment for epilepsy. The first and best-studied case of the effects on memory of bilateral removal of portions of the temporal lobes was the patient called H.M., studied by Brenda Milner, a colleague of Penfield and the surgeon William Scoville. H.M., a 27-year-old man, had suffered for over 10 years from untreatable bilateral temporal lobe seizures as a consequence of brain damage sustained at age 9 when he was hit and knocked over by someone riding a bicycle. As an adult he was unable to work or lead a normal life. At surgery the hippocampal formation, the amygdala, and parts of the multimodal association area of the temporal cortex were removed bilaterally (Figure 62-1). H.M.'s seizures were much better controlled after surgery, but the removal of the medial temporal lobes left him with a devastating memory deficit. This memory deficit (or amnesia) was quite specific. H.M. still had normal short-term memory, over seconds or minutes. Moreover, he had a perfectly good long-term memory for events that had occurred before the operation. He remembered his name and the job he held, and he vividly remembered childhood events, although he showed some evidence of a retrograde amnesia for information acquired in the years just before surgery. He retained a perfectly good command of language, including his normally varied vocabulary, and his IQ remained unchanged in the range of bright-normal. What H.M. now lacked, and lacked dramatically, was the ability to transfer new short-term memory into long-term memory. He was unable to retain for more than a minute information about people, places, or objects. Asked to remember a number such as 8414317, H.M. could repeat it immediately for many minutes, because of his good short-term memory. But when distracted, even briefly, he forgot the number. Thus, H.M. could not recognize people he met after surgery, even when he met them again and again. For example, for several years he saw Milner on an almost monthly basis, yet each time she entered the room H.M. reacted as though he had never seen her before. H.M. had a similarly profound difficulty with spatial orientation. It took him about a year to learn his way around a new house. H.M. is not unique. All patients with extensive bilateral lesions of the limbic association areas of the medial temporal lobe, from either surgery or disease, show similar memory deficits. Retrograde Amnesia—Inability to Recall Memories from the Past. When retrograde amnesia occurs, the degree of amnesia for recent events is likely to be much greater than for events of the distant past. The reason for this difference is probably that the distant memories have been rehearsed so many times that the memory traces are deeply engrained, and elements of these memories are stored in widespread areas of the brain. In some people who have hippocampal lesions, some degree of retrograde amnesia occurs along with anterograde amnesia, which suggests that these two types of amnesia are at least partially related and that hippocampal lesions can cause both. However, damage in some thalamic areas may lead specifically to retrograde amnesia without causing significant anterograde amnesia. A possible explanation of this is that the thalamus may play a role in helping the person “search” the memory storehouses and thus “read out” the memories. That is, the memory process not only requires the storing of memories but also an ability to search and find the memory at a later date. The possible function of the thalamus in this process is discussed further in Chapter 58.
• #7: Figure 62-2 The patient H.M. showed definite improvement in any task involving learning skilled movements. He was taught to trace between two outlines of a star while viewing his hand in a mirror. He improved considerably with each fresh test, although he had no recollection that he had ever done the task before. The graph plots the number of times, in each trial, that he strayed outside the outlines as he drew the star. (From Blakemore 1977.) The Distinction Between Explicit and Implicit Memory Was First Revealed With Lesions of the Limbic Association Areas of the Temporal Lobe Milner originally thought that the memory deficit after bilateral medial temporal lobe lesions affects all forms of memory equally. But this proved not to be so. Even though patients with lesions of the medial temporal lobe have profound memory deficits, they are able to learn certain types of tasks and retain this learning for as long as normal subjects. The spared component of memory was first revealed when Milner discovered that H.M. could learn new motor skills at a normal rate. For example, he learned to draw the outlines of a star while looking at his hand and the star in a mirror (Figure 62-2). Like normal subjects learning this task, H.M. initially made many mistakes, but after several days of training his performance was error-free and indistinguishable from that of normal subjects.
• #8: Figure 62-3 In a study of recall of words, amnesiacs and normal control subjects were tested under two conditions. First they were presented with common words and then asked to recall the words (free recall). Amnesiac patients were impaired in this condition. However, when subjects were given the first three letters of a word and instructed simply to form the first word that came to mind (completion), the amnesiacs performed as well as normal subjects. The baseline guessing rate in the word completion condition was 9%. (From Squire 1987.) Later work by Larry Squire and others has made it clear that the memory capacities of H.M. and other patients with bilateral medial temporal lobe lesions are not limited to motor skills. Rather, these patients are capable of various forms of simple reflexive learning, including habituation, sensitization, classical conditioning, and operant conditioning, which we discuss later in this chapter. Furthermore, they are able to improve their performance on certain perceptual tasks. For example, they do well with a form of memory called priming , in which the recall of words or objects is improved by prior exposure to the words or object. Thus, when shown the first few letters of previously studied words, a subject with amnesia correctly selects as many previously presented words as do normal subjects, even though the subject has no conscious memory of having seen the word before (Figure 62-3)
• #9: Multi-store (Atkinson-Shiffrin memory model) The multi-store model (also known as Atkinson-Shiffrin memory model ) was first recognised in 1968 by Atkinson and Shiffrin . The multi-store model has been criticized for being too simplistic. For instance, long-term memory is believed to be actually made up of multiple subcomponents, such as episodic and procedural memory. It also proposes that rehearsal is the only mechanism by which information eventually reaches long-term storage, but evidence shows us capable of remembering things without rehearsal. The model also shows all the memory stores as being a single unit whereas research into this shows different. For example, short-term memory can be broken up into different units such as visual information and acoustic information. Patient KF proves this. Patient KF was brain damaged and had problems with his short term memory. He had problems with things such as spoken numbers, letters and words and with significant sounds (such as doorbells and cats mewing). Other parts of STM were unnaffected, such as visual (pictures). It also shows the sensory store as a single unit whilst we know that the sensory store is split up into 5 different parts: taste touch visual acoustic smell SENSORY, SHORT-TERM AND LONG-TERM MEMORY In the 1960s, the distinction among various types of memory according to their duration was the subject of passionate debates. Some scientists thought that the most elegant way to account for the data available at the time was to conceptualize memory as a single system of variable duration. But bit by bit, evidence accumulated that suggested the existence of at least three distinct memory systems. Though the mechanisms of these three systems differ, they do flow naturally from one into the other and can be regarded as three necessary steps in forming a lasting memory. According to this now generally accepted model, the stimuli detected by our senses can be either ignored, in which case they disappear almost instantaneously, or perceived, in which case they enter our  sensory memory . Sensory memory does not require any conscious attention; as information is perceived, it is stored in sensory memory automatically. But sensory memory is essential, because it is what gives us the effect of unity of an object as our eyes jump from point to point on its surface to examine its details, for example. For instance, if the object in your sensory memory is a red octagon, you may or may not pay attention to it. If you do pay attention, you recognize that it is a stop sign. Once you have paid such attention to a piece of information, it can pass on to your  short-term memory . Your short-term memory lets you record limited amounts of information for periods of less than one minute. With an active effort, you can keep a piece of information in short-term memory for longer–for example, by repeating a telephone number until you have finished dialing it. Otherwise, the memory will disappear in less than a minute. Keeping an item in short-term memory for a certain amount of time lets you eventually transfer it to long-term memory for more permanent storage. This process is facilitated by the mental work of repeating the information, which is why the expression  “working memory”  is increasingly used as a synonym for short-term memory. But such repetition seems to be a less effective strategy for consolidating a memory than the technique of giving it a meaning by associating it with previously acquired knowledge. Once the piece of information has been stored in your  long-term memory , it can remain there for a very long time, and sometimes even for the rest of your life. There are, however, several factors that can make these memories hard to retrieve. These factors include how long it has been since the event stored in your memory occurred, how long it has been since the last time you remembered it, how well you have integrated it with your own knowledge, whether it is unique, whether it resembles a current event, and so on. Many experiments still need to be conducted to assess the influence of each of these factors more closely. Nevertheless, we are beginning to gain a better understanding of the underlying systems necessary for each of these three types of memory to work properly. A person’s short-term memory capacity is generally measured by the number of items they can retain when each is presented to them only once. On average, people have a short-term memory capacity of 7 items, plus or minus 2. An item can be defined as a “piece” of information. Consequently, one way to increase the storage capacity of short-term memory might be to increase the size of these pieces of information through a more effective encoding strategy, such as grouping. Here are two phenomena suggesting that there are in fact two distinct systems for short-term and long-term memory. First of all, our abilities to retain items at the start and end of a list are not equally affected by distractions. If a distraction occurs, we tend to forget the items at the end of the list (i.e., those stored in short-term memory) while remembering the ones at the start of it. In technical terms, the recency effect is attributable to short-term memory, while the primacy effect is attributable to long-term memory. Second, people with anterograde amnesia cannot form new long-term memories, but their short-term memory remains intact.
• #10: What is Sensory Memory? If you've ever twirled a lighted object (such as a flashlight or a "sparkler") in a circle at night, you've probably noticed that, if you twirled fast enough, you saw a circle of light. This is because of visual sensory memory , which is called iconic memory (the word "icon" means "image" in this context). In this case, you very briefly held in mind a mental image of the light at each point in the path being traced. If twirled fast enough, the individual iconic memories didn't have time to disappear and, therefore, they melded into a "memory circle," which you perceived as an actual circle of light. According to Baddeley (1993): This effect was used to measure the duration of the visual memory trace as long ago as 1740 by a Swedish investigator, [Johann Andreas] Segner, who attached a glowing ember to a rotating wheel. When the wheel was rotated rapidly, a complete circle could be seen, since the trace left at the beginning of the circle was still glowing brightly by the time the ember returned to its starting point. If the wheel was moved slowly, only a partial circle would be seen, since the trace of the first part had faded by the time the ember returned to its starting point. (p. 18) By varying the speed of the wheel, Segner was able to estimate the length of iconic memory to be about 1/10th of a second. The first modern study on iconic memory was performed by George Sperling (1960). In one condition, Sperling asked participants to look at a blank screen onto which he flashed very briefly (1/20th of a second) three rows of four letters each: G Z O F D H V J X R T P When asked to recall as many letters as possible, most participants named the first 4 to 5 letters. Sperling believed that each participant had stored an image of the entire set of letters but that, by the time they began to "read" the second row of their mental image, it had disappeared. In order to test the claim that iconic memory exists and that it stores a detailed visual copy of the original perception for less than a second , Sperling (1960) flashed the 12 letters to other groups of participants but asked them to recall the letters in only one of the three rows. Because it would have taken too long to orally ask them to name, say, the third row of letters (the iconic memory would have disappeared by the time he had finished), Sperling decided to sound tones of different frequencies, with each frequency indicating a particular row to to be recalled: a high-frequency tone to indicate the first row; a medium-frequency tone to indicate the middle row; and a low-frequency tone to indicate the bottom row. He found that, if the tone was sounded immediately after the letters had been flashed on the screen, most participants were able to recall all four letters in the indicated row. Sperling concluded that the participants had a complete iconic memory of the 12 letters — a memory that disappeared in a fraction of a second. Most studies of sensory memory have looked at iconic memory or echoic (auditory) memory. These studies have demonstrated that sensory memory can be described in terms of five characteristics: the level of awareness at which sensory memories occur, the duration of sensory memories, the capacity of the sensory-memory store, the encoding of sensory memories, and what causes sensory memories to be forgotten. Level of Awareness of Sensory Memories Sensory memories typically are processed at the preconscious level of awareness. For example, in Section 1-4 , you learned about the so-called cocktail-party phenomenon , in which your attention shifts to a conversation to which you were not attending after something important is said in that conversation (such as a mention of your name). The cocktail-party phenomenon occurs because your sensory memory is processing preconsciously a relatively large amount of sensory information. This processing of sensory information is constant, but it involves only superficial characteristics, such as the intensity of a sensation. The preconscious processing of information that occurs in sensory memory would fit the definition of subliminal perception , which is defined as perception that occurs below the conscious level of awareness . Thus, subliminal perception allows us to briefly store and superficially process sensory information in order to determine if it is important enough to attend to, as in the cocktail-party phenomenon. However, some have claimed that subliminal perception has complex and long-term effects on our thoughts, motives, and behaviors (see Elliston, 1999 ). For example, some have claimed that subliminal perception has such a powerful influence that it can cause us to buy products at the store (subliminal persuasion) or to improve our mental functioning (for example, to improve our memory). These claims have little or no scientific evidence to support them ( Moore, 1992 ; Phelps & Exum, 1992 ; Pratkanis, 1992 ). Although Sperling's studies as well as those of many others have shown that we have a sensory memory subsystem, which means that we engage in subliminal perception, there is no good evidence that it has a stable and pervasive influence on our behavior beyond the immediate one of shifting our attention to new stimuli. Some psychoanalysts have claimed that sensory information can be processed unconsciously and that, if it is deemed to be a threat — in the sense that it may be relevant to a repressed conflict — the information is immediately placed into the unconscious level. This process is known as "perceptual defense." It is similar to repression except that, unlike repression, in perceptual defense , information related to repressed material is transferred from the preconscious level to the unconscious level without ever entering consciousness. The evidence for this process is meager: most of the supporting evidence is based on case studies in clinical situations or experimental studies that didn't include adequate controls for extraneous variables (Holmes, 1990). Duration of Sensory Memories Sperling's (1960) research demonstrated that iconic memories are stored for a fraction of a second. Echoic memories are thought to last somewhat longer: perhaps as long as 2-3 seconds (Baddeley, 1993). Perhaps you've experienced this when hearing a police siren or a train whistle late at night that suddenly stops: you may still "hear" the echoic memory for a second or two after it has ended. Capacity of the Sensory Store When we speak of the capacity of a memory store, we are talking about the amount of information (the number of memories) it can hold at any one instant. The capacity of the sensory stores for each of the senses has not been well studied. There is some evidence suggesting that the iconic store can hold perhaps about 15-20 "icons" at any one time. Encoding of Sensory Memories Sensory memories essentially are reproductions of the original perceptions that were processed in the sensory areas of the brain. For example, an iconic memory is a detailed visual image that is similar to the original visual perception; and an echoic memory is an auditory reproduction (an "echo") of the original auditory perception. Forgetting of Sensory Memories It is thought that sensory memories consist of physiological changes in the CNS — changes that appear and disappear very rapidly. This explanation of the forgetting of sensory memories is referred to as "decay theory." The physiological or physical change underlying a memory is called an engram [ ∂ ] (also known as a "biological memory trace"; Lashley, 1950). Thus, decay theory states that the forgetting of a memory is caused by the disappearance over time of its engram . Once the engram has disappeared, the memory no longer exists anywhere in the memory system: it has "decayed." As stated, the engrams that make up sensory memories are thought to decay very rapidly. Thus, unless a sensory memory is attended to, which then transfers it to short-term memory, it will decay almost immediately. Because we can attend to only a small number of sensory memories, virtually all sensory memories at any one moment disappear within a couple of seconds at most. Thus, we forget almost every perception that we experience (preconsciously or unconsciously) during our lives.
• #11: What is Short-Term Memory? According to the SMM, when you pay attention to information in sensory memory, it gets transferred to the short-term subsystem. As was the case with sensory memory, short-term memory may be described in terms of five characteristics. Level of Awareness of Short-Term Memories Since the SMM defines attention as the mental process that transfers information into the short-term subsystem , short-term memory is, by this definition, at the conscious level of awareness. Duration of Short-Term Memories Henry M. is a famous name in the history of memory research. On September 1, 1953, he had portions of his brain removed by William Beecher Scoville — a neurosurgeon at Hartford Hospital in Hartford, Connecticut — as a treatment for his epilepsy. Sadly, after the operation, he developed profound memory problems similar to those observed in Jimmie's case (see Scoville & Milner, 1957 ): From H[enry].M.’s moment in surgery onward, every conversation for him was without predecessors, each face vague and new. Names no longer rose to the surface, neither histories nor endearing moments came any more. Reassurances of welcome had to be sought every moment from each look in every pair of eyes [because he could not remember anyone he had met since the operation].... When Dr. Scoville [the surgeon who performed the operation] came home and told his wife of the surgery, she said that he told her in the form of a joke: Guess what, I tried to cut out the epilepsy of a patient, but took his memory instead! (Hilts, 1995, p. 100) Although Henry still was able to perceive the world, to remember events for brief periods of time, to remember many episodes of his life from before the surgery, and to perform well on a standard intelligence test, he apparently had lost the ability to form new long-term memories. After his surgery, Henry was tested intensively until at least the late 1990s. The results of these tests have taught us much about the memory system. Because Henry’s short-term memory subsystem is intact, we can directly measure the duration of short-term memories by determining how long he can maintain a new memory: In one test, for example, he successfully kept the number 584 in mind for 15 minutes, and when asked how he did this, he replied, “It’s easy. You just remember 8. You see, 5, 8, 4 add to 17. You remember 8; subtract from 17 and it leaves 9. Divide 9 by half and you get 5 and 4, and there you are — 584. Easy.” Yet, a few minutes later, after his attention had shifted to something else, he could not remember the number or the mnemonic scheme[ ∂ ] he had used, or even that he had been given a number to remember. [REFERENCE?] It might seem that short-term memories can be retained for at least 15 minutes. However, Henry was maintaining the memory in the short-term store longer than usual by using maintenance rehearsal , which is a type of rehearsal [ ∂ ] in which one repeats over and over again the material to be remembered . You probably do this often during your day whenever you want to remember something for a short period of time, such as a phone number that someone has given you to dial. ( Note: You can create your own mnemonic schemes here .) In studying the duration of short-term memories, researchers prevent study participants from using maintenance rehearsal by asking them to perform a mental task, such as counting backward from 100 by 3’s, immediately after giving them material to memorize, such as a word list. After a short period of time (say, 5 seconds), the researchers ask the participants to recall as much of the material as possible. By varying the amount of time that passes before recall, memory researchers can determine how quickly a memory is lost from the short-term store. Such research has demonstrated that we begin forgetting short-term memories within the first few seconds after receiving new information (see Figure 3). By about 15 seconds after receiving the information, any memory of it has virtually disappeared from the short-term store. Although we can increase the duration of short-term memories with maintenance rehearsal (as Henry M. did in the passage quoted above), the memories will disappear very rapidly when we stop repeating the information. Therefore, if our memory system contained only the short-term store and the mental process of maintenance rehearsal, our memories would be virtually useless for most purposes. Capacity of the Short-term Store The amount of information that can be held in the short-term store at any one time is easily measured. For example, we can present word lists with varying numbers of words and determine at what point people start to have trouble: 3 Items:  grand    bear       top 4 Items:   kite        hive        core       wean 5 Items:   rod         ice          week      gate       pin 6 Items:   ear         axe         zoo         lake       joke        vase 7 Items:   fine         dime      cake       nice       moat      shell       oar 8 Items:   lint         pine       year       urn         bore       zinc        air         mine 9 Items:   sea         time       oak         earn       wine      care        jam       like        born 10Items:  vine       nook      rare        yarn       lawn      bone      hook      wear      grain     door You can take a test that measures the capacity of your short-term memory for lists of letters here . And you can take another test that measures the capacity of your short-term memory for pictures here . Once the number of items to be remembered gets larger than about 6-8 items, people begin to have difficulty maintaining the entire list in their short-term store. On average, people are able to hold about seven items in their short-term store at any given time, give or take a couple of items ( Miller, 1956 ). Encoding of Short-term Memories Encoding involves various ways of processing (manipulating and transforming) information in order to mentally label it — to form a memory code for the information. How we encode depends on the nature of the material to be remembered and what we are trying to do with this information. In looking at encoding in short-term memory, most studies have used rapidly presented verbal material (such as a word list or a list of nonsense syllables) that people then immediately recall. Under these conditions, people tend to use mostly maintenance rehearsal to memorize the material. Let’s say that we have presented the following list of words to a group of people: cat   run   suit   junk   frame   clothes   dress   stone   age   log   boat   watch We probably will find that, although most people will be able to recall several of these words, they also may remember incorrectly some words that were not presented. These misremembered words can tell us what kind of encoding a person used. For example, at the extreme, someone might incorrectly remember hearing the following 12 words: bat   bun   fruit   bunk   lame   droves   mess   bone   rage   bog   goat   botch It is very improbable that any one person would incorrectly remember all these words; but an individual might make one or more of these mistakes. Now, in what way are these misremembered words similar to the original list? If you repeat the words out loud, you will find that the misremembered words rhyme with the original list of words. These mistakes suggest that the original list of words was phonemically encoded in short-term memory. In other words, when verbal material is encoded for storage in short-term memory, it tends to be encoded according to the way it sounds. If the words had been semantically encoded, what kinds of mistakes would we have observed when subjects recalled the list? In this case, the mistakes would have contained words such as the following: feline   jog   tuxedo   trash   border   shirts   skirt   rock   old   wood   ship   clock If we encoded information semantically in short-term memory, people would misremember words that have a meaning similar to those in the original list. The fact that this rarely happens shows that we do not often use semantic encoding for the initial formation of short-term memories. We will talk more about the encoding of short-term memories when the topic of working memory is discussed in Section 4-3 . Forgetting of Short-Term Memories Earlier in this section, decay theory was defined as, "the forgetting of a memory ... caused by the disappearance over time of its engram." When an engram disappears, the memory no longer exists: it has "decayed." A memory subsystem that is limited in terms of the duration of its memories contains engrams that decay over a period of time. In the case of short-term memories, the engrams decay within 20 seconds unless maintenance rehearsal is used. Thus, the neural changes that form the foundation for short-term memory degrade over a short period of time . Another reason why short-term memories are quickly lost involves the limited capacity of the short-term store. Once this capacity is met, the addition of new information requires that information already in the store be "pushed out." Displacement theory states that the forgetting of memories occurs when new information pushes old information out of the memory store . Because the short-term store can hold only about seven items of information, displacement is an important cause of forgetting from this memory subsystem .
• #12: Working memory The working memory model. In 1974 Baddeley and Hitch proposed a working memory model which replaced the concept of general short term memory with specific, active components. In this model, working memory consists of three basic stores: the central executive, the phonological loop and the visuo-spatial sketchpad. In 2000 this model was expanded with the multimodal episodic buffer. [3] The central executive essentially acts as attention. It channels information to the three component processes: the phonological loop, the visuo-spatial sketchpad, and the episodic buffer. The phonological loop stores auditory information by silently rehearsing sounds or words in a continuous loop; the articulatory process (the "" over and over again), then a list of short words is no easier to remember. The visuo-spatial sketchpad stores visual and spatial information. It is engaged when performing spatial tasks (such as judging distances) or visual ones (such as counting the windows on a house or imagining images). The episodic buffer is dedicated to linking information across domains to form integrated units of visual, spatial, and verbal information and chronological ordering (e.g., the memory of a story or a movie scene). The episodic buffer is also assumed to have links to long-term memory and semantical meaning. The working memory model explains many practical observations, such as why it is easier to do two different tasks (one verbal and one visual) than two similar tasks (e.g., two visual), and the aforementioned word-length effect. However, the concept of a central executive as noted here has been criticized as inadequate and vague Working Memory Is a Short-Term Memory Required for Both the Encoding and Recall of Explicit Knowledge How is explicit memory recalled and brought to consciousness? How do we put it to work? Both the initial encoding and the ultimate recall of explicit knowledge (and perhaps some forms of implicit knowledge as well) are thought to require recruitment of stored information into a special short-term memory store called working memory. As we learned in Chapter 19, working memory is thought to have three component systems Box 62-1 The Transformation of Explicit Memories How accurate is explicit memory? This question was explored by the psychologist Frederic Bartlett in one series of studies in which the subjects were asked to read stories and then retell them. The recalled stories were shorter and more coherent than the original stories, reflecting reconstruction and condensation of the original. The subjects were unaware that they were editing the original stories and often felt more certain about the edited parts than about the unedited parts of the retold story. The subjects were not confabulating; they were merely interpreting the original material so that it made sense on recall. Observations such as these lead us to believe that explicit memory, at least episodic (autobiographical) memory, is a constructive process like sensory perception. The information stored as explicit memory is the product of processing by our perceptual apparatus. As we saw in earlier chapters, sensory perception itself is not a faithful record of the external world but a constructive process in which incoming information is put together according to rules inherent in the brain's afferent pathways. It is also constructive in the sense that individuals interpret the external environment from the standpoint of a specific point in space as well as from the standpoint of a specific point in their own history. As discussed in Chapter 25, optical illusions nicely illustrate the difference between perception and the world as it is. Moreover, once information is stored, later recall is not an exact copy of the information originally stored. Past experiences are used in the present as clues that help the brain reconstruct a past event. During recall we use a variety of cognitive strategies, including comparison, inferences, shrewd guesses, and suppositions, to generate a consistent and coherent memory.
• #13: The working memory itself is located in the prefrontal cortex. As experimental techniques became refined, it has become clear that there is no rigid dividing line between a memory and a thought. A model of working memory has been developed to combine perceptions, memories and concepts together, and consists of three parts: Phonological loop - Memory in this area (see Figure 25) enables us to remember sequences of approximately seven digits, letter, or words. The language areas of the brain are mainly in the left hemisphere, around and above the ear. The language loop start with hearing words in the auditory cortex and/or reading words in the visual cortex. Perception of language results from the convergence of auditory and visual information in Wernicke's area. Expression of language is controlled by Broca's area; while the angular gyrus is concerned with meaning. Visual-spatial scratch pad - It is like a sort of inner eye, which receives and codes data into visual or spatial images. For example, it comes into play when we need to remember where we were on a page when we start reading a book again. Functional imaging suggests that this complex structure represents the "what" and "where" in short-term memory (see Figure 25). Central executive - This most important yet least well understood component of the working memory model, is postulated to be responsible for the selection, initiation, and termination of processing routines (e.g., encoding, storing, retrieving). It is believed that this component coordinates information from a number of sources, directs the ability to focus and switch attention, organizes incoming material and the retrieval of old memories and combines information arriving via the other two temporary storage systems. It performs various tasks such as reasoning or doing mental arithmetic - rather like the RAM (Radom Access Memory) of a computer. An attentional control system (or central executive ), thought to be located in the prefrontal cortex (Chapter 19), actively focuses perception on specific events in the environment. The attentional control system has a very limited capacity (less than a dozen items). The attentional control system regulates the information flow to two rehearsal systems that are thought to maintain memory for temporary use: the articulatory loop for language and the visuospatial sketch pad for vision and action. The articulatory loop is a storage system with a rapidly decaying memory trace where memory for words and numbers can be maintained by subvocal speech. It is this system that allows one to hold in mind, through repetition, a new telephone number as one prepares to dial it. The visuospatial sketch pad represents both the visual properties and the spatial location of objects to be remembered. This system allows one to store the image of the face of a person one meets at a cocktail party. The information processed in either one of these rehearsal, working memory systems has the possibility of entering long-term memory. The two rehearsal systems are thought to be located in different parts of the posterior association cortices. Thus, lesions of the extrastriate cortex impair rehearsal of visual imagery whereas lesions in the parietal cortex impair rehearsal of spatial imagery.
• #14: Consolidation of Memory For short-term memory to be converted into long-term memory that can be recalled weeks or years later, it must become “consolidated.” That is, the short-term memory if activated repeatedly will initiate chemical, physical, and anatomical changes in the synapses that are responsible for the long-term type of memory. This process requires 5 to 10 minutes for minimal consolidation and 1 hour or more for strong consolidation. For instance, if a strong sensory impression is made on the brain but is then followed within a minute or so by an electrically induced brain convulsion, the sensory experience will not be remembered. Likewise, brain concussion, sudden application of deep general anesthesia, or any other effect that temporarily blocks the dynamic function of the brain can prevent consolidation. Consolidation and the time required for it to occur can probably be explained by the phenomenon of rehearsal of the short-term memory as follows. How Do Working Memory and Long-term Memory Interact? Much of the research that has led to our current understanding of the components of working memory and the characteristics of the short-term store made use of memory tasks involving the rapid visual or auditory presentation of numbers, words, or letters, followed almost immediately (within about 15 seconds) by retrieval. In this research, therefore, the encoding and storing of information in working memory made use of the phonological loop and maintenance rehearsal — a type of rehearsal that produces primarily phonemic memory codes[∂]. In everyday life, however, verbal material that is important to us and that we want to remember for a while (such as the material in this book) typically is not memorized in this way. For instance, when you study for a test, you are unlikely to receive a high score if you rapidly read the textbook material and then immediately take the test. Instead, those who get high scores typically go back over the material many times before taking the test. In addition, there typically is a relatively long time period (definitely more than 15 seconds) between the last time they study for the test and the retrieval of the material during the test. In memory research, this is called the retention interval , which is defined as the elapsed time between the encoding (and storing) of material, and the retrieval of this material (such as on a test). Phonemic encoding of verbal material typically does not produce long-lasting memory codes, which is why students who consistently do well on tests do not rely solely on maintenance rehearsal when studying. Instead, they have learned that thinking deeply about the information and relating it to other things they already know is the best way to receive high test scores. In this case, they are using their working memories to semantically encode[∂] the material. If your goal is to create enduring long-term memories of the material — memories that can be activated in many ways, then it is best to semantically encode the material. Semantic memory codes are produced through elaborative rehearsal — a type of information processing that links new information to information already maintained in the long-term store . It should be obvious that it is easier to remember something you are learning if you can associate it with something that you already know. For example, I can easily remember the name of anyone named "Jeff" because that is my name, which is a name that obviously is already stored in my long-term memory along with strong emotions related to that name. In the case of emotionally neutral words, such as retrieval , elaborative rehearsal would include methods of information processing such as the following: Forming a visual image of the word "retrieval," perhaps by imagining your hand pulling a word out of your opened head. Relating "retrieval" to a past experience, such as thinking of the time that you suddenly remembered the name of your first-grade teacher after being unable to recall it at first. Defining "retrieval" in your own words, such as "retrieval means getting information out of a memory store." Of course, there are other ways to elaboratively rehearse this term. Any method that allows you to transform the information into something meaningful would be an example of elaborative rehearsal. Let's use elaborative rehearsal to memorize the six-item word list presented in Section 4-2: ear, axe, zoo, lake, joke, vase . One way to do this would be to create a story out of the words: A man cut off the ear of a stuffed animal with an axe at the zoo down by the lake . As a joke , he put it in a vase and gave it to his friend. ” As you can see, the story does not have to be a good one, or even one that makes much sense. It only has to be meaningful to you. In this case, the meaning involved organizing the words into an ordered sequence. We also could have elaboratively rehearsed these items by visualizing each one located in a different spot in an imagined scene. As very popular method of elaborative rehearsal is chunking . A chunk is a meaningful unit of information . For example, what at first may seem to be a random sequence of letters or numbers sometime may be chunked into a smaller number of meaningful items: n   b   c   c   i   a   t   g   i   f   b   m   w These thirteen unrelated letters may be organized into four chunks of meaningful information nbc            cia            tgif            bmw Four chunks is a number that can be held in the short-term store easily by almost anyone. By elaborating with chunking, the total amount of information held in the short-term store has been dramatically increased. Furthermore, by forming meaningful memory codes, it becomes more likely that the information will be transferred to the long-term store, where it should form enduring memories (see below). The process of elaborative rehearsal shows clearly that working memory must be interacting constantly with the long-term subsystem: in creating semantic memory codes, the information in the short-term store is being linked to information held in the long-term store. An example reported by Baddeley (1993) illustrates well this interaction between the short-term and long-term stores: John Bransford describes an informal experiment in which experimenter E [let's call him "Ed"] walked into the office of a colleague C [let's call him "Craig"] and said simply, ‘Bill has a red car’. [This statement was made with no explanation.] Here is the description of Craig's reactions: ‘Craig looked very surprised, paused for about three seconds, and finally exclaimed “What the hell are you talking about?” After a hasty de-briefing session Craig laughed and told Ed what had gone on in his head. First he thought that Ed was talking about a person named Bill that Craig knew. Then Craig realized that Ed could not in all probability know that person; and besides Bill would never buy a red car. Then Craig thought that Ed may have mixed up the name and really meant to say Jeff (a mutual friend of Craig and Ed). Craig knew that Jeff had ordered a new car, but he was surprised that it was red and that it had arrived so soon. Craig also entertained a few additional hypotheses — all within about three seconds. After that he gave up, thereupon uttering “What the hell are you talking about?”’ (pp. 143-144; modified slightly from the original) As you can see from this example and from your own experiences, our working memory constantly takes in information from the outside world and relates it to other information already in long-term memory. This process of elaboratively rehearsing the new information occurs very rapidly. By comparing in working memory the new information with information already in the long-term store, we often are able to make sense of the new information, thereby forming semantic codes. (See this link to learn how to study better for tests using the findings of memory research.)
• #15: Rehearsal Enhances the Transference of Short-Term Memory into Long-Term Memory. Psychological studies have shown that rehearsal of the same information again and again in the mind accelerates and potentiates the degree of transfer of short-term memory into longterm memory and therefore accelerates and enhances consolidation. The brain has a natural tendency to rehearse newfound information, especially newfound information that catches the mind’s attention. Therefore, over a period of time, the important features of sensory experiences become progressively more and more fixed in the memory stores. This explains why a person can remember small amounts of information studied in depth far better than large amounts of information studied only superficially. It also explains why a person who is wide awake can consolidate memories far better than a person who is in a state of mental fatigue.
• #16: New Memories Are Codified During Consolidation. One of the most important features of consolidation is that new memories are codified into different classes of information. During this process, similar types of information are pulled from the memory storage bins and used to help process the new information.The new and old are compared for similarities and differences, and part of the storage process is to store the information about these similarities and differences, rather than to store the new information unprocessed. Thus, during consolidation, the new memories are not stored randomly in the brain but are stored in direct association with other memories of the same type. This is necessary if one is to be able to “search” the memory store at a later date to find the required information.
• #17: How Much Information Can We Remember? Because attention and elaborative rehearsal typically take time and effort, and because attention and elaborative rehearsal are necessary for remembering much of the information entering our sensory memory during a single episode of our lives, we can expect to forget almost everything that we experience from one episode to the next. Most of the information entering sensory memory is never attended to and, therefore, is never transferred to the short-term store. Only a fraction of the transferred information is elaborated well enough to be transferred on to the long-term store. Figure 5 presents an illustration of the amount of information transferred from one store to the next during a single episode (the amount of information in a store is indicated by its relative size): Figure 5. The Amount of Information Transferred From One Memory Store to Another During a Single Life Episode. On the other hand, the more that we learn about a particular topic, the less time and effort it takes to elaboratively rehearse new information related to that topic. This is because, when there exists a large amount of related information in the long-term store, there exist more memories to which new information can be linked. In fact, when you become an expert on a topic, elaborative rehearsal for new information related to that topic occurs almost automatically. For example, Schacter (1996) discussed the ability of chess experts to automatically memorize the positions of pieces on a chess board after simply glancing at the board: After just a single five-second exposure to a board from an actual game, international [chess] masters in one study remembered the locations of nearly all twenty-five pieces, whereas novices could remember the locations of only about four pieces. Moreover, it did not matter whether the masters knew that their memory for the board would be tested later; they performed just as well when they glanced at the board with no intention to remember it . (p. 48; emphasis added) Chess grandmasters have encoded and stored in long-term memory an extraordinary amount of information about past chess games (both those games they have played themselves and those games played by other chess masters they have studied); and, therefore, they are able to compare rapidly and effortlessly a current game with their long-term memories of prior games. This feat, however, does not indicate a superior memory ability: they were no better than anyone else at remembering the positions of randomly placed pieces (Ericsson & Lehmann, 1996; Ross, 2006). In other words, the chess pieces had to be in a meaningful relation to each other on the board in order for chess grandmasters to demonstrate their ability to rapidly and effortlessly encode and store the positions of the pieces in long-term memory: To a beginner, a position with 20 chessmen on the board may contain far more than 20 chunks of information, because the pieces can be placed in so many configurations. A grandmaster, however, may see one part of the position as "fianchettoed bishop in the castled kingside," together with a "blockaded king's-Indian-style pawn chain," and thereby cram the entire position into perhaps five or six chunks. (Ross, 2006, p. 69) The moral of this research for your academic work is clear: the more that you learn about an academic area (such as psychology) , the less time and effort is required to elaboratively rehearse and remember new information in that area.
• #18: Long-Term Memory There is no obvious demarcation between the more prolonged types of intermediate long-term memory and true long-term memory. The distinction is one of degree. However, long-term memory is generally believed to result from actual structural changes , instead of only chemical changes, at the synapses, and these enhance or suppress signal conduction. Again, let us recall experiments in primitive animals (where the nervous systems are much easier to study) that have aided immensely in understanding possible mechanisms of long-term memory. Structural Changes Occur in Synapses During the Development of Long-Term Memory Electron microscopic pictures taken from invertebrate animals have demonstrated multiple physical structural changes in many synapses during development of long-term memory traces. The structural changes will not occur if a drug is given that blocks DNA stimulation of protein replication in the presynaptic neuron; nor will the permanent memory trace develop. Therefore, it appears that development of true long-term memory depends on physically restructuring the synapses themselves in a way that changes their sensitivity for transmitting nervous signals. The most important of the physical structural changes that occur are the following: 1. Increase in vesicle release sites for secretion of transmitter substance. 2. Increase in number of transmitter vesicles released. 3. Increase in number of presynaptic terminals. 4. Changes in structures of the dendritic spines that permit transmission of stronger signals. Thus, in several different ways, the structural capability of synapses to transmit signals appears to increase during establishment of true long-term memory traces. Memory Based on Chemical Changes in the Presynaptic Terminal or Postsynaptic Neuronal Membrane Figure 57–9 shows a mechanism of memory studied especially by Kandel and his colleagues that can cause memories lasting from a few minutes up to 3 weeks in the large snail Aplysia . In this figure, there are two synaptic terminals. One terminal is from a sensory input neuron and terminates directly on the surface of the neuron that is to be stimulated; this is called the sensory terminal. The other terminal is a presynaptic ending that lies on the surface of the sensory terminal, and it is called the facilitator terminal . When the sensory terminal is stimulated repeatedly but without stimulation of the facilitator terminal, signal transmission at first is great, but it becomes less and less intense with repeated stimulation until transmission almost ceases. This phenomenon is habituation , as was explained previously. It is a type of negative memory that causes the neuronal circuit to lose its response to repeated events that are insignificant. Conversely, if a noxious stimulus excites the facilitator terminal at the same time that the sensory terminal is stimulated, then instead of the transmitted signal into the postsynaptic neuron becoming progressively weaker, the ease of transmission becomes stronger and stronger; and it will remain strong for minutes, hours, days, or, with more intense training, up to about 3 weeks even without further stimulation of the facilitator terminal. Thus, the noxious stimulus causes the memory pathway through the sensory terminal to become facilitated for days or weeks thereafter. It is especially interesting that even after habituation has occurred, this pathway can be converted back to a facilitated pathway with only a few noxious stimuli.
• #19: What is Long-Term Memory? According to the cognitive approach, the long-term subsystem consists only of the long-term store. In other words, cognitive theorists do not include any components in this subsystem that further process information transferred from working memory. The long-term store can be described in terms of the same characteristics used to describe the sensory and short-term stores: the levels of awareness at which long-term memories are stored; the duration of long-term memories; the capacity of the long-term store; the encoding of long-term memories; and the causes of forgetting of long-term memories. The first characteristic will be discussed in this section. The other characteristics and the topic of forgetting from the long-term store will be discussed in Section 4-4 . Levels of Awareness of Long-Term Memories We can think of the ability to attend to information stored in the memory system as involving a continuum from conscious, to preconscious, to unconscious (see Figure 6). By definition, only memory codes in the short-term store are at the conscious level. Therefore, memory codes in the long-term store are below the conscious level. As you probably know from your own experience, some long-term memories are relatively easy to retrieve (that is, to bring to the conscious level), such as the day and time of your favorite television show. These memory codes are stored at the preconscious[ ∂ ] level. On the other hand, other long-term memories are much more difficult, perhaps even impossible, to retrieve, such as the name of your second-grade teacher. These memory codes are stored at the unconscious[ ∂ ] level. In order to bring long-term memories to the conscious level, long-term memory codes must be attended to, which leads to their transfer to the short-term store. Unconscious long-term memory codes may not be able to enter the short-term store, but they still may be transferred to unconscious components of working memory, thereby affecting conscious cognitions, emotions, and behaviors. A long-term memory code stored at the preconscious level is called an explicit memory . For example, your conscious memory of what you were doing five minutes ago involves the activation of a preconscious long-term memory code. Or, the activation of a preconscious memory code about the meaning of the term "explicit memories" may allow you to answer a test question about explicit memories correctly. A long-term memory code stored at the unconscious level is called an implicit memory . For example, an implicit memory of the meaning of the term "implicit memories" may cause you to "guess" correctly when responding to a test item about the concept. Or an implicit memory of being frightened when you were an infant by a psychologist banging a hammer against a steel bar ( Watson & Rayner, 1920 ) may cause you to avoid taking psychology courses in college. Figure 6. Levels of Awareness & the Ease or Difficulty of Attending to Mental Content at Each Level The case of Henry M. can help us to understand better the distinction between explicit and implicit memories. As we saw in Section 4-2 , it was very difficult for Henry to learn new information. Scoville and Milner (1957) provided some examples of Henry's amnesia: Ten months ago [Henry's] family moved from their old house to a new one a few blocks away on the same street; he still has not learned the new address, though remembering the old one perfectly, nor can he be trusted to find his way home alone. Moreover, he does not know where objects in continual use are kept; for example, his mother still has to tell him where to find the lawn mower, even though he may have been using it only the day before. She also states that he will do the same jigsaw puzzles day after day without showing any practice effect and that he will read the same magazine over and over again without finding their contents familiar. This patient has even eaten luncheon in front of one of us [Brenda Milner] without being able to name, a mere half-hour later, a single item of food he had eaten; in fact, he could not remember having eaten luncheon at all. (p. 14) Nevertheless, there are some things that he could learn and remember quite well. For example, he was tested often over many years in the same room at the Massachusetts Institute of Technolog y. When walking down the hall to the testing room, he would claim that he did not know where this room was, yet he would make the correct turns taking him to it. He seemed to know approximately where the room was, but he did not know that he knew this! This suggests that his amnesia was not complete. Although Henry did not have an explicit memory for the location of the testing room, he did have an implicit memory, which allowed him o walk there. In general, Henry was unable to form new explicit memories, but he seemed able to form new implicit memories. Another example of this can be seen in a case study from a century ago (described in Sacks, 1995). One day in 1911, a neurologist by the name of Edouard Claparéde took his medical students on rounds. One of Claparéde's patients was a man with severe anterograde amnesia similar to Henry's. In order to demonstrate the man's memory disturbance, Claparéde placed a pin between his fingers and, when he reached for the patient's hand to shake it, he instead stuck the pin in the man's palm. Within a minute or two after this event, the man was unable to remember why his hand hurt: he had no episodic memory of the incident. The next day, however, the patient, although unable to recall the incident, refused to shake hands with Claparéde. This means that the patient must have formed an implicit memory of the incident. In fact, this patient’s reaction involved the classical conditioning of a fear response (the CS was the sight of Claparéde’s hand, the UCS was the pain caused by the pin, and the CR was the fear elicited by the sight of Claparéde’s hand). Classical conditioning often involves the formation of implicit memories. In order for a person's long-term memory code (whether explicit or implicit) to affect his or her conscious cognitions, emotions, and behaviors, the person must experience a retrieval cue , which is a stimulus that activates a long-term memory code. If the memory code is an explicit one, the retrieval cue will cause a conscious memory to be retrieved. If the memory code is an implicit one, the retrieval cue will cause changes in conscious cognitions, emotions, and/or behaviors (as in the case of Claparéde’s patient) . Retrieval cues activate long-term memory codes because they are, in some way, associated with them. For example, if you want to retrieve explicit memories of the names of your friends in third grade, standing in your third-grade classroom or seeing a class picture from third grade might be adequate retrieval cues. In fact, these retrieval cues may bring back a flood of memories from that time period. Or perhaps, while eating waffles one day, you suddenly think of a friend from childhood. As you think more about this friend, you may remember the time you stayed overnight at his house and ate waffles for breakfast. In this example, the waffles were a retrieval cue because they were associated with this explicit memory of your childhood friend. Smells and tastes seem to be especially good retrieval cues for explicit memories of life events from long ago. For example, Marcel Proust (1871–1922), the famous French novelist and essayist, described in a passage from his novel, In Search of Lost Time ( À la Recherche du Temps Perdu ), the recall of an intense and stirring childhood memory: The other evening, having come in chilled, by the snow, and not being able to get warm, as I had started to read in my room under the lamp, my old cook offered to make tea, which I never drink. And chance had it that she brought me some slices of toast. I dipped the toast into my mouth and having the sensation against my palate of its sogginess permeated with the taste of tea, I felt a disturbance, scents of geraniums and orange trees, a feeling of extraordinary light, of happiness; I stayed motionless, fearing a single movement could interrupt what was happening in me, which I did not understand, but still concentrating on this taste of dunked bread, which seemed to produce such wonders, when suddenly the shaken partitions of my memory caved in, and it was the summers I spent in the country house I mentioned which burst into my consciousness, with their mornings, and drawing with them the procession, the nonstop charge of happy hours. Then I remembered: every day when I was dressed, I went down into the room of my grandfather, who had just awakened me and was drinking his tea. He used to dunk a rusk and give it to me. (quoted in Hilts, 1995, p. 77) As an adult, the taste of toast dipped in tea served as a retrieval cue for a memory not recalled in many years — a memory of summers spent at a relative’s house. The memory's powerful impact was due to its vivid perceptual details and the strong emotions evoked. Most remembrances, however, are not so clear and striking. Retrieval cues (such as the several choices you read on a question from a multiple-choice test) are more likely to activate memories of general knowledge or fuzzy memories of past life events. Another example of a retrieval cue involves the common experience of having to return to the place in which you recently had thought of something in order to remember what that thought was. For example, perhaps while you were in your bedroom one afternoon, you decided to drive to the grocery store and pick up a few things. You remembered that your car keys were in the kitchen and, so, you started to walk down the hallway towards the kitchen. Halfway there, however, you forgot why you were going to the kitchen. At that point, it is likely that you stopped, turned around, and returned to the bedroom. When you looked around the bedroom, you probably remembered suddenly that you had been walking to the kitchen to get your car keys. In this example, your bedroom served as a retrieval cue for the memory. The concept of retrieval cue helps to explain why using elaborative rehearsal to encode information in the short-term store is the best strategy for creating easily accessible and stable long-term memories. When we elaboratively encode information, a memory code is created that can be activated by a larger number of retrieval cues because elaborative rehearsal creates a number of links to information already stored in the long-term subsystem. Furthermore, the greater the number of possible retrieval cues, the more likely it becomes that the memory will be retrieved often, which creates a stronger and more durable memory code. For example, if you encoded the name of the behaviorist, "John Watson" in terms of his later career as an advertising executive (see here ), you may be unable to answer a test question asking you to "name the famous behaviorist who studied the conditioning of fear in Little Albert." A memory code created through the use of elaborative rehearsal, however, probably would allow you to answer this question easily since a larger number of retrieval cues (such as this test question) would activate the memory code associated with John Watson. Retrieval cues may also explain déjà vu experiences , in which people have the uncanny feeling that they have already experienced in the past the situation in which they find themselves currently. The cognitive theory of déjà vu states that stimuli in the current situation are acting as a retrieval cue for a memory of a different, but still similar, situation from the past. For example, when looking out at a classroom of faces, teachers sometimes feel as if they are reliving a previous experience. It is likely, however, that the current situation is activating a long-term memory code of a past experience in which they were looking out at a classroom full of faces, especially if that experience occurred in the same classroom. (A comedian once stated that he was suffering from two memory problems — amnesia and déjà vu —at the same time. He said, “I feel as if I’ve forgotten this before.”) There is evidence that mental states can serve as retrieval cues. For example, state-dependent memory is said to occur when people are better able to retrieve information learned while in a particular mental state (such as a state of consciousness or a mood state) when they are again in that same mental state . For example, if you study for your test while drinking a few beers, you may do better on the test if you have a few beers while taking it. Before you put this strategy into action, however, you should be aware that you will do much better on the test if you are sober both while studying for the test and while taking the test. Nonetheless, some research on state-dependent memory suggests that, if you are slightly intoxicated while studying, then you may do a bit better if you are slightly intoxicated while taking the test. A mood is a stable (lasts for at least several days) and pervasive (occurs in most situations) emotional state . For example, in order to be diagnosed with a mood disorder, such as major depression , a person must experience the emotion of depression (severe sadness) every day for most of the day over at least a two-week period. A person who becomes depressed for a day after receiving a low grade on a chemistry test is experiencing a change in emotions, but not changes in moods. Some research on state-dependent memory suggests that the mood you are in while learning material may serve as a retrieval cue when you later are asked to remember this information. For example, if you learn a word list while depressed, then you may retrieve more of this information if you are depressed than if you are in a neutral or a happy mood. Thus, if you study for a test while depressed and anxious, you may do better on the test if you are depressed and anxious while taking it. Again, you probably will perform best if you are happy and energetic both while studying for the test and while taking the test. A concept related to but distinct from state-dependent memory is the mood-congruence effect . The mood-congruence effect is an increased tendency to recall life events consistent with one's present mood relative to life events inconsistent with that mood . For example, when you are happy, you are more likely to recall positive experiences from your past (such as the time you found a twenty-dollar bill) than you are to recall negative experiences (such as the time you lost a twenty-dollar bill). In other words, research on the mood-congruence effect suggests that your present mood state may serve as a retrieval cue for long-term memories formed when you were in the same mood state. The mood-congruence effect implies that it may not be a good idea to trust the negative memories of a depressed, anxious, or distressed person: such a person would be expected to more easily recall negative than positive long-term memories. If you have known this person for a long time, you may be amazed at how many positive memories he or she seems to be forgetting. There is no need to worry, however. As soon as the person's mood improves, there should be an increase in the number of positive memories retrieved. Furthermore, when you are feeling anxious and depressed, you are likely to remember past events as being more negative than than you remember them at other times: Researchers have documented ... that people tend naturally to reconstruct the past in terms of their present circumstances, exaggerating the degree of earlier misfortune and trauma if they currently are feeling bad, minimizing it if they are feeling good. Findings from various studies — of Gulf War veterans, car accident victims, witnesses to school shootings, international peacekeepers — are remarkably consistent in this regard. They show that individuals with more severe symptoms of anxiety and depression remember a traumatic event as being worse when they are asked about it a second time many months, or even years, after the first. Those with fewer symptoms, however, tended to recall the event as less harrowing than they had previously described it. (Sommers & Satel, 2005, p. 155) In other words, ..... This is somewhat different than the mood-congruence effect, though related to it.
• #20: What Are the Main Characteristics of Long-Term Memory? At the end of Section 4-3 , you learned that long-term memories are stored at the preconscious and unconscious levels. In this section, you will learn about other major characteristics of the long-term subsystem: duration, capacity, and memory codes. Duration of Long-Term Memories By definition, long-term memory codes last a relatively long time. In research situations, we typically are interested in long-term memory codes that last at least a few hours to a few weeks. When trying to estimate the duration of long-term memory codes, however, we run into problems that did not occur when estimating the duration of short-term memory codes. Short-term memories are easily accessed because their memory codes are stored at the conscious level, whereas long-term memories, because their memory codes are stored at the preconscious and unconscious levels, can be accessed only with the proper retrieval cues. This fact makes it very difficult to measure accurately the average duration of long-term memory codes: a person may not retrieve a long-term memory because its memory code no longer exists (the engram has disappeared) or because, although the memory code still exists, the retrieval cue used to activate it was inadequate (that is, it was not associated closely enough with the memory to activate the code). Thus, it is virtually impossible to estimate the average duration of long-term memory codes, especially when they are implicit. A retrieval task is defined by the type of retrieval cue used to activate a long-term memory code . In research on explicit memories, which involve memory codes stored at the preconscious level, two kinds of retrieval task often are used: recall tasks and recognition tasks . In a recall task , the retrieval cue is simply a request to retrieve items of information learned at an earlier time . In other words, no real retrieval cue is given. For example, in the section on working memory, we saw that memory researchers often use lists of numbers, words, or letters that are read aloud to study participants and then recalled immediately. Let's say that the following word list is used: pen  cow  bar  man  day  few  hot Because this list has only seven items — which is the average capacity of the short-term store — and the recall is immediate, which prevents the decay of short-term memory codes (a possibility made less likely when subjects use maintenance rehearsal to hold the information in the short-term store), most participants will easily recall all seven words.A recall task is much more difficult, however, when one is trying to retrieve long-term memories. This is because, in order to retrieve a long-term memory, a retrieval cue is needed; and no retrieval cue is provided in a recall task. Thus, study participants must mentally construct their own retrieval cues. For example, let's say you are asked to recall the names of all U.S. presidents from 1945 to the present. Assuming that you have, at some time or another, learned this information and stored it in the long-term subsystem, you must search for it in the long-term store and activate the relevant memory codes. In recalling the names, many of you probably began your search by thinking of the current president (George Bush) and working your way backwards: Bill Clinton, George Bush, Ronald Reagan ... at which point you may have begun to have trouble. Perhaps you recalled that Richard Nixon and John Kennedy came before Ronald Reagan, but you may not have been able to recall the remaining presidents. Constructing a retrieval cue yourself (that is, searching for the information and activating the memory codes) involves complex mental processes that require a great deal of mental effort for memories that are only vaguely recalled at first. This is why many of you find essay tests to be so difficult: they are pure recall tasks that ask you to retrieve information that is difficult and probably incompletely learned. In a recognition task , the retrieval cue consists of items of information learned at an earlier time. The learner is asked to recognize the items that he or she was exposed to in the past. For example, police often use photographic or physical line-ups, which are made up of five to seven people, one of which is a suspect in a crimiknal investigation. An eyewitness to the crime looks at the faces in the hope that he or she will recognize the suspect as the perpetrator of the crime. Or a week after memorizing the word list presented above, you may be given the following list of words and asked to recognize the items learned the previous week: oak  pen  hat  cow  bar  man  arm  big  gun  day  old  few  fog  hot  pit People typically find it much easier to recognize items that they learned a week before than to recall those items. This is because they don't need to search through their long-term store for the correct information and then activate the corresponding memory codes. Instead, the previously learned items in the list directly activate the memory codes. Recognition tasks often used in school tests are multiple-choice items in which the correct choice is worded identically to information learned when studying.In research on implicit memories, which involve memory codes stored at the unconscious level, recall and recognition tasks cannot be used. Instead, two other kinds of retrieval task often are used in memory research: relearning tasks and priming tasks . In a relearning task , the retrieval cue consists of exposure to forgotten items of previously learned information that then are relearned more quickly and with less effort . For example, people who have taken a year or two of a second language in high school often cannot consciously recall or recognize much of it when they take it again a couple of years later in college. Nevertheless, many people relearn much more quickly what they had learned previously. The fact that they relearned the information more quickly the second time around can be explained only by concluding that unconscious memory codes were stored in the long-term subsystem. In a priming task , a person is exposed to a "prime" (typically a word or image) that is forgotten over time; and later is given a retrieval cue consisting of a portion of the prime, which is quickly processed and responded to in a way related to the prime. For example, let's say that study participants are asked to use the following words to construct a story: lint  pine  year  turn  bore  zinc  pair  mine A few weeks later, it is likely that they will neither recall nor recognize these words since they had not been asked to memorize them. These words would serve as the prime in this study. In order to show that the prime caused them to develop implicit memory codes, the study participants might be given the following list of incomplete words and asked to fill in the spaces: l_n_   p__e   ye__   t_r_   b__e   _i_c   pa__   m_n_ People given such a task tend to complete the partial words with the words they were exposed to several weeks before. Again, the only way this could occur is if the prime had caused the development of implicit memory codes. In general, the priming effect refers to a tendency to respond automatically to a stimulus in a particular way after prior exposure to a similar stimulus ; and it probably affects us in many ways in our everyday lives. For example, it may be that many of the “great ideas” we have — ideas that seem to be caused by some mysterious power of intuition — actually are caused by priming. That is, we may have heard an idea before but no longer have an explicit memory for it. The implicit memory for the idea, however, may cause us to interpret events in such a way that this idea now “pops into our heads.” For example, a key idea in Freud’s development of his psychoanalytic approach seems to have arisen through priming: Freud had maintained for years an intense and tumultuous friendship with the Berlin physician Wilhelm Fliess. He frequently confided his latest ideas and insights to Fliess, and was emotionally dependent on his approval of them. When Freud announced to Fliess a momentous new insight — that every person is fundamentally bisexual — he fully expected Fliess to be amazed by the idea. Instead, Fliess responded by reminding Freud that he himself had made exactly the same discovery two years earlier and [had] told Freud all about it, and that Freud had rejected the idea. (Schacter, 1996, p. 168) Freud eventually remembered the earlier event and realized his mistake. In many cases, however, it is likely that people do not remember the priming event. In fact, you may have experienced such a disagreement yourself, and become very frustrated when the other person did not remember events the way you did. If you learn nothing else about memory in this chapter, you should at least learn that our memory can be very faulty. In fact, we probably should not place much trust in many explicit long-term memories without looking for objective evidence that the memory is relatively accurate. Capacity of the Long-Term Store It is believed that we can store an unlimited amount of information in the long-term store. That is, in the course of a lifetime, it seems quite probable that we can never fill up the long-term store. Everyday experience should tell you that this claim is probably true. For example, elderly people who have accumulated a great deal of information in their long-term stores over a lifetime can still learn new facts (such as the name of a person they have just met) without losing old memories. And no matter how much you have studied for the large number of tests you have had over the years, it is not likely that you will ever reach a point where you cannot learn even more information: you will not fill up your long-term store with the material you have learned in your previous courses. We also can look at this issue in terms of adaptation — we would not be well-adapted creatures if we could ever get to a point where we reach the capacity of our long-term stores. Such a human would not be able to learn new information and, hence, would not be expected to survive much longer as conditions changed. The Encoding of Long-Term Memories When encoding for long-term memory, the proper rehearsal of information in short-term memory is essential. As discussed earlier, the processing of information in short-term memory can occur in either a shallow or a deep manner. Maintenance rehearsal represents a shallow processing of information because you simply are repeating the information over and over again with very little effort being expended on transforming and organizing the material. On the other hand, elaborative rehearsal represents a deep processing of information because you are using effort to link the new information up with older information already stored in long-term memory. As I have already discussed, research shows that the more deeply we process new information in short-term memory, the more likely it is that it will be encoded for transfer to long-term memory. When we investigate how memories for word lists are encoded for long-term storage, we often find evidence that people make use of elaborative rehearsal. For example, if we have people memorize any one of the word lists presented earlier, and then test their memories for it a day or two later, we probably will find that, when they make mistakes, they will substitute words that have a similar meaning. That is, if I asked you to memorize the following word list: lint  pine  year  turn  bore  zinc  pair  mine and then tested your long-term memory for it two days from now, you are likely to make mistakes such as the following: dust  tree  hour  spin  dull iron  join  bomb Although you probably will remember some of the words correctly, when you do make a mistake, it is likely to involve a word that has a meaning associated with the original word, which implies that you had semantically encoded the information for long-term storage. In order to semantically encode verbal information, you must have used elaborative rehearsal. In general, studies such as this show that elaborative rehearsal leads to the transfer of the greatest amount of information from short-term to long-term memory as well as to the most stable long-term memories.
• #21: What Does Organic Amnesia Tell Us About Long-Term Memory? Some of the earliest evidence in support of the claim that the hippocampus is important in memory formation can be found in the case study of Henry M. — a case that you first learned about in Section 4-2 .  In 1953, the hippocampus (as well as the amygdala and some neighboring structures in the cortex) on each side of Henry's brain were removed because of very severe epilepsy that had not responded to medication. As soon as he woke up from the operation, however, Henry exhibited severe anterograde amnesia involving explicit long-term memories. To be specific, he could remember recent events for only about 30 seconds (Hilts, 1995). For example: He was unable to learn his way around the hospital after the operation. Soon after reading something, he could remember neither what he had read nor that he had read it. After moving to a new house, it took him eight years to learn how to get from one room to another. He never learned to find his way back to the house from a distance of more than two blocks. When he was moved into a nursing home in 1980, he never learned where he lived or who cared for him. Henry’s memory problems involved mainly the ability to form explicit memories, especially those involving life events. On the other hand, he seemed to have little or no trouble forming new implicit memories, especially those involving new behaviors and skills. For example, Henry learned to read words written backwards, to solve particular puzzles, and to walk to the room in which he was tested each year at MIT . The fact that he could perform these tasks correctly demonstrates that Henry had implicit memories for the corresponding skills. Nevertheless, he did not remember that he knew how to perform these tasks: he had not formed explicit memories of having learned the skills. For example, when walking to the testing room, he would state that he did not know where he was going or why he was walking in that direction. Although Henry's anterograde amnesia seemed complete, research in later years showed that he was able to recall explicit memories of some events that had occurred after 1953: For example, Henry seems to remember something about a President Kennedy and an assassination, even though the killing occurred ten years after Henry’s surgery. But ... you couldn’t say he remembers the assassination of John Kennedy exactly. ... If asked about an assassinated president he will remember McKinley, and recall an attempt on Franklin Roosevelt’s life. If Dallas is mentioned, he may say, “Kennedy!” Asked for a first name, he may say Ted. Then, asked to reconstruct the Kennedy assassination, he will speak of an assassin who was trying to shoot the president but someone else got in the way; this is a description of the killing of Chicago mayor Anton Cermak, who was killed in 1933 by a bullet intended for Franklin Roosevelt. ... It seems as if there is some leakage from the world into his memory, some vague impressions which have become something like memories. (Hilts, 1995, p. 113) To a small degree, Henry was able to develop semantic memories, which are explicit memories that consist of general knowledge about an object, event, activity, or situation. For example, fill in the following blanks: (a) Abraham Lincoln was the ___th president of the United States ; (b) ______ was the first person to step on the moon ; (c) William McKinley was shot in the city of ______ . When you study for a test, you are trying to develop semantic memories. You probably also have semantic memories of your social security number, your telephone number, your birth date, and your street address. When you remember events from your own life, on the other hand, you are retrieving episodic memories, which are explicit memories of events, activities, and situations that include the memory of one's self participating in them. In other words, episodic memories are memories of personal experiences. For example, memories of what you did ten minutes ago or of your first day of kindergarten are episodic memories. When you tell someone what you had for dinner last night, who was there, and what time it was at, you are recalling an episodic memory as long as you remember your own participation in the dinner. People with damage to their hippocampi have more trouble storing and retrieving episodic memories than they do with storing and retrieving semantic memories. A good example of this can be found in Oliver Sack's (1995) case study of Greg, first described in Section 1-1 . In 1991, Sacks took Greg to a concert featuring Greg's favorite band, the Grateful Dead . Because Greg’s amnesia extended back to the late 1960s, he had no semantic or episodic memories of music performed by the Grateful Dead since that time. At the concert, Greg seemed to enjoy himself but was confused when the "new" music was performed: The first half of the concert had many earlier pieces, songs from the sixties, and Greg knew them, loved them, joined in. His energy and joy were amazing to see, he clapped and sang nonstop, with none of the weakness and fatigue he generally showed. ... But the second half of the concert was somewhat strange for Greg: more of the songs dated from the mid- or late seventies and had lyrics that were unknown to him, though they were familiar in style. He enjoyed these, clapping and singing along wordlessly, or making up words as he went. ... The newer songs [from 1980 and after] went far beyond any development that he could have imagined, were so beyond (and in some ways so unlike) what he associated with the Dead, that it “blew his mind.” It was, he could not doubt, “their” music he was hearing, but it gave him an almost unbearable sense of hearing the future. (Sacks, 1995, pp. 75-76) By the next morning, Greg no longer remembered having been at the concert or having heard any of the newer songs: his episodic memories for the concert had decayed from working memory. Nevertheless, he sang some of the new songs he had heard for the first time the night before! That is, he had developed semantic memories for them. This suggests that the hippocampi (along with the amygdala and parts of the cerebral cortex) are more involved with the encoding and storing of new episodic memories than of new semantic memories. In Figure 2, a summary of the components of the long-term subsystem are illustrated. DIFFERENT TYPES OF LONG-TERM MEMORY As the diagram below shows, long-term memory can be divided into  explicit and implicit memory , and implicit memory can in turn be divided into  various subtypes . But always bear in mind that in the actual workings of human memory, these various subsystems are interacting all the time. The interactions between episodic and semantic memory-two distinct forms of explicit memory-may offer the best example (see sidebar). Episodic memory  (sometimes called autobiographical memory) lets you remember events that you personally experienced at a specific time and place. It includes memories such as the meal you ate last night, or the name of an old classmate, or the date of some important public event. The most distinctive feature of episodic memory is that you see yourself as an actor in the events you remember. You therefore memorize not only the events themselves, but also the entire context surrounding them. Episodic memory is the kind most often affected by various forms of amnesia. Also, the emotional charge that you experience at the time of the events conditions the quality of your memorization of the episode. Semantic memory  is the system that you use to store your knowledge of the world. It is a knowledge base that we all have and much of which we can access quickly and effortlessly. It includes our memory of the meanings of words–the kind of memory that lets us recall not only the names of the world’s great capitals, but also social customs, the functions of things, and their colour and odour. Semantic memory also includes our memory of the rules and concepts that let us construct a mental representation of the world without any immediate perceptions. Its content is thus abstract and relational and is associated with the meaning of verbal symbols. Semantic memory is independent of the spatial/temporal context in which it was acquired. Since it is a form of reference memory that contains information accumulated repeatedly throughout our lifetimes, semantic memory is usually spared when people suffer from amnesia, but it can be affected by some forms of dementia (see sidebar). Semantic memory can be regarded as the residue of experiences stored in episodic memory. Semantic memory homes in on common features of various episodes and extracts them from their context. A gradual transition takes place from episodic to semantic memory. In this process, episodic memory reduces its sensitivity to particular events so that the information about them can be generalized. Conversely, our understanding of our personal experiences is necessarily due to the concepts and knowledge stored in our semantic memory. Thus, we see that these two types of memory are not isolated entities, but rather interact with each other constantly. In Alzheimer’s disease, patients quickly develop difficulty in retrieving individual words and general knowledge. Studies have shown that in tasks such as describing and naming items, these patients display a loss of knowledge of the specific characteristics of semantic categories. Initially, they lose the ability to distinguish fine categories, such as species of animals or types of objects. But over time, this lack of discrimination extends to broader, more general categories. At first, if you show such patients a spaniel, they may say, “that is a dog”. Later, they may just say “that is an animal”.
• #22: Semantic, and episodic memory are the subclasses of declarative memory: Episodic memory - It is about an event in one's life and everything about it, including emotional reactions. Remembering an episode, e.g., the attack on Pearl Harbour (Figure 28), is to create a memory for a unique event that only happened once and there is no opportunity for learning the event by rehearsal. Episodic memories are not very reliable, they are highly personal, selective, idiosyncratic and varying over time, but they may also be richly complex and movie-like in character. They constitute the stories we tell ourselves about our past, they are the things we would write about in our autobiography. Episodic memories can be recalled deliberately or are triggered by evocative sensory stimuli - particularly by the sense of smell. Episodic memory involves the use of the hippocampus for forming memories and the cortex for storage (see diagram D, Figure 24 ). Semantic memory - Semantic memory is the knowledge of facts - numbers, addresses, language and concepts - which the brain files in categories and which seems to involve the left temporal lobe. Retrieval is then carried out by the frontal lobes (see diagram E, Figure 24 ). We assume all of the facts that constitute our knowledge of things must be stored in an organized fashion to be useful. Though this has not been demonstrated, it seems likely that the brain stores our semantic memories as modules that have some logical links to one another; they are grouped by category for instance. On retrieval, the brain knows where to find the memory according to the address of that particular category. Semantic memory is essential to the understanding of how things work and thus to an under-standing of the world we live in. It is a body of knowledge that helps us to regulate our behaviour according to and dependent on reliable factual memories. Navigational skills, for example, depend on our ability to deploy a complex store of semantic memory, including detailed spatial memories and representations of the world.
• #23: Declarative memory - Declarative memory covers the memory of facts such as events and names, which do not need to be repeated for them to sink in. Those experiences destined to be laid down as long-term memories are shunted down to the hippocampus where they are held in storage for 2 - 3 years. During this time the hippocampus replays the experiences back up to the cortex, and each rehearsal etches it deeper into the cortex. Eventually the memories are so firmly established in the cortex that the hippocampus is no longer needed for their retrieval. Much of the hippocampal replay is thought to happen during sleep. Dreams consist partly of a rerun of things that have happened during the day, fired up to the cortex by the hippocampus. The visual areas generate rerun of daily sightings (Figure27). LONG-TERM MEMORY Recent research has provided a complex, highly intricate picture of memory functions and their loci in the brain.  The hippocampus , the temporal lobes, and  the structures of the limbic system  that are connected to them are essential for the consolidation of long-term memory. The hippocampus facilitates associations among various parts of the cortex, for example, between a tune that you heard at a dinner party and the faces of the other guests who were at the table. However, all other things being equal, such associations would naturally fade over time, so that your mind did not become cluttered with useless memories. What might cause such associations to be strengthened and eventually etched into long-term memory very often depends on “limbic” factors, such as  how interested you were in the occasion, or what emotional charge  it may have had for you, or how gratifying you found its content. The various structures of the limbic system exert their influence on the hippocampus and the temporal lobe via Papez’s circuit, also known as the hippocampal/mammillothalamic tract. This circuit is a sub-set of the numerous connections that the limbic structures have with one another. The diagram here shows the route that information travels from the hippocampus to the mammillary bodies of the hypothalamus, then on to the anterior thalamic nucleus, the cingulate cortex, and the entorhinal cortex, before finally returning to the hippocampus. Once the temporary associations of cortical neurons generated by a particular event have made a certain number of such “passes” through Papez’s circuit, they will have undergone a physical remodelling that consolidates them. Eventually, these associations will have been strengthened so much that they will stabilize and become independent of the hippocampus. Bilateral lesions of the hippocampus will prevent new long-term memories from forming, but will not erase those that were encoded before the injury. With this gradual disengagement of the limbic system, the memories will no longer pass through Papez’s circuit, but instead will be encoded in specific areas of the cortex: the same ones where the sensory information that created the memories was initially received (the occipital cortex for visual memories, the temporal cortex for auditory memories, etc.).
• #24: Figure 62-5 The anatomical organization of the hippocampal formation. A. The key components of the medial temporal lobe important for memory storage can be seen in the medial (left) and ventral (right) surface of the cerebral hemisphere. B. The input and output pathways of the hippocampal formation. Animal Studies Help to Understand Memory The surgical lesion of H.M.'s temporal lobe encompassed a number of regions, including the temporal pole, the ventral and medial temporal cortex, the amygdala, and the hippocampal formation (which includes the hippocampus proper, the subiculum, and the dentate gyrus) as well as the surrounding entorhinal, perirhinal, and parahippocampal cortices. Since lesions restricted to any one of these several sectors of the medial temporal lobe are rare in humans, experimental lesion studies in monkeys have helped define the contribution of the different parts of the temporal lobe to memory formation. Mortimer Mishkin and Squire produced lesions in monkeys identical to those reported for H.M. and found defects in explicit memory for places and objects similar to those observed in H.M. Damage to the amygdala alone had no effect on explicit memory. Although the amygdala stores components of memory concerned with emotion (Chapter 50), it does not store factual information. In contrast, selective damage to the hippocampus or the polymodal association areas in the temporal cortex with which the hippocampus connects—the perirhinal and parahippocampal cortices—produces clear impairment of explicit memory. Thus, studies with human patients and with experimental animals suggest that knowledge stored as explicit memory is first acquired through processing in one or more of the three polymodal association cortices (the prefrontal, limbic, and parieto-occipital-temporal cortices) that synthesize visual, auditory, and somatic information. From there the information is conveyed in series to the parahippocampal and perirhinal cortices, then the entorhinal cortex, the dentate gyrus, the hippocampus, the subiculum, and finally back to the entorhinal cortex. From the entorhinal cortex the information is sent back to the parahippocampal and perirhinal cortices and finally back to the polymodal association areas of the neocortex (Figure 62-5). Thus, in processing information for explicit memory storage the entorhinal cortex has dual functions. First, it is the main input to the hippocampus. The entorhinal cortex projects to the dentate gyrus via the perforant pathway and by this means provides the critical input pathway through which the polymodal information from the association cortices reaches the hippocampus (Figure 62-5B). Second, the entorhinal cortex is also the major output of the hippocampus. The information coming to the hippocampus from the polymodal association cortices and that coming from the hippocampus to the association cortices converge in the entorhinal cortex. It is therefore understandable why the memory impairments associated with damage to the entorhinal cortex are particularly severe and why this damage affects not simply one but all sensory modalities. In fact, the earliest pathological changes in Alzheimer disease, the major degenerative disease that affects explicit memory storage, occurs in the entorhinal cortex.
• #25: Figure 62-6 The role of the hippocampus in memory. We spend much of our time actively moving around our environment. This requires that we have a representation in our brain of the external environment, a representation that can be used to find our way around. The right hippocampus seems to be importantly involved in this representation, whereas the left hippocampus is concerned with verbal memory. A. The right hippocampus is activated during learning about the environment. These scans were made while subjects watched a film that depicted navigation through the streets of an Irish town. The activity during this task was compared with that in the control task where the camera was static and people and cars came by it. In the latter case there was no learning of spatial relationships and the hippocampus was not activated. Areas with significant changes in activity, indexed by local perfusion change, are indicated in yellow and orange. The scan on the left is a coronal section and the scan on the right is a transaxial section; in each panel the front of the brain is on the right and the occipital lobe on the left. (From Maguire et al. 1996.) B. The right hippocampus also is activated during the recall by licensed taxi drivers of routes around the city of London. These people spend a long time learning the intricacies of the road network in the city and are able to describe the shortest routes between landmarks as well as the names of the various streets. The right parahippocampal and hippocampal regions are significantly activated when they do this task. The scan on the left is a coronal section and the scan on the right is a transaxial section; in each panel the front of the brain is on the right and the occipital lobe on the left. Areas with significant changes in activity, indexed by local perfusion change, are depicted in yellow and orange. (From Maguire et al. 1996.) C. Three anatomical slices in the coronal (left upper), transverse (right upper), and sagittal (right lower) planes show activation (red) in the left hippocampus associated with the successful retrieval of words from long lists that have to be memorized. A = anterior, P = posterior, I = inferior. Damage Restricted to Specific Subregions of the Hippocampus Is Sufficient to Impair Explicit Memory Storage Given the large size of the hippocampus proper, how extensive does a bilateral lesion have to be to interfere with explicit memory storage? Clinical evidence from several patients, as well as studies in experimental animals, suggests that a lesion restricted to any of the major components of the system can have a significant effect on memory storage. For example Squire, David Amaral, and their collegues found that the patient R.B. had only one detectable lesion after a cardiac arrest—a destruction of the pyramidal cells in the CA1 region of the hippocampus. Nevertheless, R.B. had a defect in explicit memory that was qualitatively similar to that of H.M., although quantitatively it was much milder. The different regions of the medial temporal lobe may, however, not have equivalent roles. Although the hippocampus is important for object recognition, for example, other areas in the medial temporal lobe may be even more important. Damage to the perirhinal, parahippocampal, and entorhinal cortices that spares the underlying hippocampus produces a greater deficit in memory storage, such as object recognition than do selective lesions of the hippocampus that spare the overlying cortex On the other hand, the hippocampus may be relatively more important for spatial representation. In mice and rats lesions of the hippocampus interfere with memory for space and context, and single cells in the hippocampus encode specific spatial information (Chapter 63). Moreover, functional imaging of the brain of normal human subjects shows that spatial memories involve more intense hippocampal activity in the right hemisphere than do memories for words, objects, or people, while the latter involve greater activity in the hippocampus in the dominant left hemisphere. These physiological findings are consistent with the finding that lesions of the right hippocampus give rise to problems with spatial orientation, whereas lesions of the left hippocampus give rise to defects in verbal memory (Figure 62-6).
• #26: Connections between the hippocampus and possible memory storage sites. The rhesus monkey brain is shown because these connections are much better documented in subhuman primates than in humans. Projections to this region are shown in (A); the efferent projections from the hippocampus are shown in (B). Projections from numerous cortical areas converge on the hippocampus and the related structures known to be involved in human memory; most of these sites also send projections to the same cortical areas. Medial and lateral views are shown, the latter rotated 180° for clarity. (After Van Hoesen, 1982.) The Long-Term Storage of Information Revealing though they have been, clinical studies of amnesic patients have provided relatively little insight into the long-term storage of information in the brain (other than to indicate quite clearly that such information is  not  stored in the midline diencephalic structures that are affected in anterograde amnesia). Nonetheless, a good deal of circumstantial evidence implies that the cerebral cortex is the major long-term repository for many aspects of memory. One line of evidence comes from observations of patients undergoing electroconvulsive therapy (ECT). Individuals with severe depression are often treated by the passage of enough electrical current through the brain to cause the equivalent of a full-blown seizure (this procedure being done under anesthesia in well-controlled circumstances). This remarkably useful treatment was discovered because depression in epileptics was perceived to be alleviated after a spontaneous seizure. However, ECT often causes both anterograde and retrograde amnesia. The patients typically do not remember the treatment itself or the events of the preceding days, and their recall of events of the previous 1–3 years can also be affected. Animal studies (rats tested for maze learning, for example) have confirmed the amnesic consequences of ECT. The memory loss usually clears over a period of weeks to months. However, to mitigate this side effect (which may be the result of excitotoxicity; see  Box B  in  Chapter 6 ), ECT is often delivered to only one hemisphere at a time. The nature of amnesia following ECT supports the conclusion that long-term memories are widely stored in the cerebral cortex, since this is the part of the brain predominantly affected by this therapy. Since different cortical regions have different cognitive functions (see  Chapters 26  and  27 ), it is not surprising that these sites store information that reflects the cognitive function of the relevant part of the brain. For example, the lexicon that links speech sounds and their symbolic significance is located in the association cortex of the superior temporal lobe, since damage to this area typically results in an inability to link words and meanings (Wernicke's aphasia; see  Chapter 27 ). Presumably, the widespread connections of the hippocampus to the language areas serve to consolidate declarative information in these and other language-related cortical sites ( Figure 31.7 ). By the same token, the inability of patients with temporal lobe lesions to recognize objects and/or faces suggests that such memories are stored in that location. Similarly, frontal lobe syndromes imply that memories about appropriate behaviors in a given social context and future plans reside in the frontal cortex (see  Chapter 26 ). With respect to procedural learning, the motor skills gradually acquired through practice are evidently stored in the basal ganglia, cerebellum, and premotor cortex (see  Chapters 17 – 19 ). Thus lesions of these sites cause a loss of the ability to make complex coordinated movements that can be considered a sort of “motor amnesia.” This scheme for long-term information storage is diagrammed in  Figure 31.8 , although the generality of the diagram only emphasizes the rudimentary state of present thinking about exactly how and where long-term memories are stored. A reasonable guess is that each complex memory is instantiated in the activity of an extensive network of neurons whose triggering depends on synaptic weightings that have been molded and modified by experience
• #27: Explicit Memory Is Stored in Association Cortices Lesions of the medial temporal lobe in patients such as H.M. and R.B. interfere only with the long-term storage of new memories. These patients retain a reasonably good memory of earlier events, although with severe lesions such as those of H.M. there appears to be some retrograde amnesia for the years just before the operation. How does this come about? The fact that patients with amnesia are able to remember their childhood, the lives they have led, and the factual knowledge they acquired before damage to the hippocampus suggests that the hippocampus is only a temporary way station for long-term memory. If so, long-term storage of episodic and semantic knowledge would occur in the unimodal or multimodal association areas of the cerebral cortex that initially process the sensory information For example, when you look at someone's face, the sensory information is processed in a series of areas of the cerebral cortex devoted to visual information, including the unimodal visual association area in the inferotemporal cortex specifically concerned with face recognition (see Box 28-1 and Chapter 28). At the same time, this visual information is also conveyed through the mesotemporal association cortex to the parahippocampal, perirhinal, and entorhinal cortices, and from there through the perforant pathway to the hippocampus. The hippocampus and the rest of the medial temporal lobe may then act, over a period of days or weeks, to facilitate storage of the information about the face initially processed by the visual association area of the inferotemporal lobe. The cells in the visual association cortex concerned with faces are interconnected with other regions that are thought to store additional knowledge about the person whose face is seen, and these connections could also be modulated by the hippocampus. Thus the hippocampus might also serve to bind together the various components of a richly processed memory of a person. Viewed in this way the hippocampal system would mediate the initial steps of long-term storage. It would then slowly transfer information into the neocortical storage system. The relatively slow addition of information to the neocortex would permit new data to be stored in a way that does not disrupt existing information. If the association areas are the ultimate repositories for explicit memory, then damage to association cortex should destroy or impair recall of explicit knowledge that is acquired before the damage. This is in fact what happens. Patients with lesions in association areas have difficulty in recognizing faces, objects, and places in their familiar world. Indeed, lesions in different association areas give rise to specific defects in either semantic or episodic memory.
• #28: Semantic (Factual) Knowledge Is Stored in a Distributed Fashion in the Neocortex As we have seen, semantic memory is that type of long-term memory that embraces knowledge of objects, facts, and concepts as well as words and their meaning. It includes the naming of objects, the definitions of spoken words, and verbal fluency. How is semantic knowledge built up? How is it stored in the cortex? The organization and flexibility of semantic knowledge is both remarkable and surprising. Consider a complex visual image such as a photograph of an elephant. Through experience this visual image becomes associated with other forms of knowledge about elephants, so that eventually when we close our eyes and conjure up the image of an elephant, the image is based on a rich representation of the concept of an elephant. The more associations we have made to the image of the elephant, the better we encode that image, and the better we can recall the features of an elephant at a future time. Furthermore, these associations fall into different categories. For example, we commonly know that an elephant is a living rather than a nonliving thing, that it is an animal rather than a plant, that it lives in a particular environment, and that it has unique physical features and behavior patterns and emits a distinctive set of sounds. Moreover, we know that elephants are used by humans to perform certain tasks and that they have a specific name. The word elephant is associated with all of these pieces of information, and any one bit of information can open access to all of our knowledge about elephants As this example illustrates, we build up semantic knowledge through associations over time. The ability to recall and use knowledge—our cognitive efficiency —is thought to depend on how well these associations have organized the information we retain. As we first saw in Chapter 1, when we recall a concept it comes to mind in one smooth and continuous operation. However, studies of patients with damage to the association cortices have shown that different representations of an object—say, different aspects of elephants—are stored separately. These studies have made clear that our experience of knowledge as a seamless, orderly, and cross-referenced database is the product of integration of multiple representations in the brain at many distinct anatomical sites, each concerned with only one aspect of the concept that came to mind. Thus, there is no general semantic memory store; semantic knowledge is not stored in a single region. Rather, each time knowledge about anything is recalled, the recall is built up from distinct bits of information, each of which is stored in specialized (dedicated) memory stores. As a result, damage to a specific cortical area can lead to loss of specific information and therefore a fragmentation of knowledge.
• #29: Episodic (Autobiographical) Knowledge About Time and Place Seems to Involve the Prefrontal Cortex Whereas some lesions to multimodal association areas interfere with semantic knowledge, others interfere with the capacity to recall any episodic event experienced more than a few minutes previously, including dramatic personal events such as accidents and deaths in the family that occurred before the trauma. Remarkably, patients with loss of episodic memory still have the ability to recall vast stores of factual (semantic) knowledge. One patient could remember all personal facts about his friends and famous people, such as their names and their characteristics, but could not remember any specific events involving these individuals. The areas of the neocortex that seem to be specialized for long-term storage of episodic knowledge are the association areas of the frontal lobes. These prefrontal areas work with other areas of the neocortex to allow recollection of when and where a past event occurred (Chapter 19). A particularly striking symptom in patients with frontal lobe damage is their tendency to forget how information was acquired, a deficit called source amnesia. Since the ability to associate a piece of information with the time and place it was acquired is at the core of how accurately we remember the individual episodes of our lives, a deficit in source information interferes dramatically with the accuracy of recall of episodic knowledge.
• #30: Nondeclarative memory - Nondeclarative memory includes skill learning, implicit learning, priming, simple classical conditioning, and habituation. These forms of learning are similar in that it is experience which changes the neural makeup, and the conscious access to past episodes is not essential for the formation of these memories. Implicit memory is not flexible and does not allow for the recombination of learned information. Nondeclarative memory does not require the hippocampus or related structures. Instead, the implicit learning of skills and habits depends on the neostriatum (basal ganglia and its connections to the frontal lobes). The conditioning simple skeletal muscle reactions depends on the cerebellum. The amygdala is essential for emotional conditioning. Nondeclarative memory can be classified to five main groups: Procedural memory - It is the repository of such skills as handwriting or driving. These skills are essential part of our memory store, but it is difficult to describe the "know-how" in words. In this sense the memory is said to be implicit or non-declarative (Figure 26); you just cannot explain how to ride a bicycle. The skills may be difficult to acquire, but once learnt they are never forgotten, even without occasional practice. Thus it seems that the know-ledge or information required for the execution of very complex motor routines or procedures is somehow laid down in a robust permanent memory store. The parts of the brain involved in the acquisition of complex motor skills are the cerebellum and putamen (see Figure 24 ). Deeply ingrained habits are stored in the caudate nucleus . Classical conditioning - Along with motor skills, conditioning is part of non-declarative memory. The desire for food at a particular time of day - regardless of whether hungry or not - is one example of such conditioning. A classical example is to associate the ring of a bell to food when feeding a dog. After repeating the training many times, the dog shows salivation at the ring of the bell even without food (see Figure 26). Fear memory - Recent study in delivering shocks to mice suggests that fear memory does not occur immediately after a painful event; rather, it takes time for the memory to become part of our consciousness. The initial event activates NMDA receptors - molecules on cells that receive messages and then produce specific physiological effect in the cell - which are normally quiet but triggered when the brain receives a shock. Over time, the receptors leave their imprint on brain cells. A phobia is an excessive or unreasonable fear of an object, place or situation. Examples include fears of specific things such as insect, snake, mouse, and flying. It seems that people can learn to suppress a fright reaction by repeatedly confronting, in a safe manner, the fear-triggering memory or stimulus. It is found that for specific phobias, up to 90% of people can be cured through such exposure therapy. Nonassociative memory - Nonassociative memory includes two forms of learning called habituation and sensitization. Habituation is defined as a decreased in response to a repeated stimulus such as a certain odor. On the other hand, sensitization is an increased responsiveness such as more sensitive in touching a cut in the skin. Nonassociative learning involves reflex pathways in the spinal cord and elsewhere. Remote memory - The memory of events that occurred in the distant past is referred to as remote memory. The underlying anatomy of remote memory is poorly understood, in part because testing this type of memory must be personalized to a patient’s autobiographical past. What is known is that, like semantic memory, remote memory eventually becomes independent of the hippocampus. One memory model shows a linear representation of how experience is processed as memory: Stimulus Sensory Registration Attention Short Term Memory Consolidation - Retrieval Long Term Memory Remote Memory. At the stage of sensory registration, there is a matching/assigning of the pattern to a meaning. Short-term memory is temporary and is limited in space. If short-term memory is not repeated, the information is lost fairly quickly. Long term memory is consolidated and stored throughout the nervous system. Remote memories represent the foundation memories upon which more recent memories are built. Since early acquired information is the foundation for new memories and may be linked to many more new memories, such memory is less subject to change and/or loss. Similar to the short-term memory, the remote memories are not usually affected by aging. Implicit Memory Can Be Nonassociative or Associative Psychologists often study implicit forms of memory by exposing animals to controlled sensory experiences. Two major procedures (or paradigms) have emerged from such studies, and these have identified two major subclasses of implicit memory: nonassociative and associative. In nonassociative learning the subject learns about the properties of a single stimulus. In associative learning the subject learns about the relationship between two stimuli or between a stimulus and a behavior. Nonassociative learning results when an animal or a person is exposed once or repeatedly to a single type of stimulus. Two forms of nonassociative learning are common in everyday life: habituation and sensitization. Habituation is a decrease in response to a benign stimulus when that stimulus is presented repeatedly. For example, most people are startled when they first hear the sound of a firecracker on the Fourth of July, Independence Day in the United States, but as the celebration progresses they gradually become accustomed to the noise. Sensitization (or pseudoconditioning ) is an enhanced response to a wide variety of stimuli after the presentation of an intense or noxious stimulus. For example, an animal responds more vigorously to a mild tactile stimulus after it has received a painful pinch. Moreover, a sensitizing stimulus can override the effects of habituation, a process called dishabituation. For example, after the startle response to a noise has been reduced by habituation, one can restore the intensity of response to the noise by delivering a strong pinch. Sensitization and dishabituation are not dependent on the relative timing of the intense and the weak stimulus; no association between the two stimuli is needed. Not all forms of nonassociative learning are as simple as habituation or sensitization. For example, imitation learning, a key factor in the acquisition of language, has no obvious associational element. Two forms of associative learning have also been distinguished based on the experimental procedures used to establish the learning. Classical conditioning involves learning a relationship between two stimuli, whereas operant conditioning involves learning a relationship between the organism's behavior and the consequences of that behavior.
• #31: Implicit Memory Is Stored in Perceptual, Motor, and Emotional Circuits Unlike explicit memory, implicit memory does not depend directly on conscious processes nor does recall require a conscious search of memory. This type of memory builds up slowly, through repetition over many trials, and is expressed primarily in performance, not in words. Examples of implicit memory include perceptual and motor skills and the learning of certain types of procedures and rules. Different forms of implicit memory are acquired through different forms of learning and involve different brain regions. For example, memory acquired through fear conditioning, which has an emotional component, is thought to involve the amygdala. Memory acquired through operant conditioning requires the striatum and cerebellum . Memory acquired through classical conditioning, sensitization, and habituation—three simple forms of learning we shall consider later—involves charges in the sensory and motor systems involved in the learning . Implicit memory can be studied in a variety of perceptual or reflex systems in either vertebrates or invertebrates. Indeed, simple invertebrates provide useful models for studying the neural mechanisms of implicit learning.
• #32: Certain Forms of Implicit Memory Involve the Cerebellum and Amygdala Lesions in several regions of the brain that are important for implicit types of learning affect simple classically conditioned responses. The best-studied case is classical conditioning of the protective eyeblink reflex in rabbits, a specific form of motor learning. A conditioned eyeblink can be established by pairing an auditory stimulus with a puff of air to the eye, which causes an eyeblink. Richard Thompson and his colleagues found that the conditioned response (eyeblink in response to a tone) can be abolished by a lesion at either of two sites. Damage to the vermis of the cerebellum , even a region as small as 2 mm2 abolishes the conditioned response, but does not affect the unconditioned response (eyeblink in response to a puff of air). Interestingly, neurons in the same area of the cerebellum show learning-dependent increases in activity that closely parallel the development of the conditioned behavior. Second, a lesion in the interpositus nucleus , a deep cerebellar nucleus, also abolishes the conditioned eyeblink. Thus, both the vermis and the deep nuclei of the cerebellum play an important role in conditioning the eyeblink, and perhaps other simple forms of classical conditioning involving skeletal muscle movement. Maseo Ito and his colleagues have shown that the cerebellum is involved in another form of implicit memory. The vestibulo-ocular reflex keeps the visual image fixed by moving the eyes when the head moves (Chapter 41). The speed of movement of the eyes in relation to that of the head (the gain of the reflex) is not fixed but can be modified by experience. For example, when one first wears magnifying spectacles, eye movements evoked by the vestibulo-ocular reflex are not large enough to prevent the image from moving across the retina. With experience, however, the gain of the reflex gradually increases and the eye can again track the image accurately. As with eyeblink conditioning, the learned changes in the vestibulo-ocular reflex require not only the cerebellum (the flocculus) but also one of the deep cerebellar nuclei (the vestibular) in the brain stem (see Chapters 41 and 42). Finally, as we have seen in Chapter 50, lesions of the amygdala impair conditioned fear.
• #34: Brain size as a function of age. The human brain reaches its maximum size (measured by weight in this case) in early adult life and decreases progressively thereafter. This decrease presumably represents the gradual loss of neural circuitry in the aging brain, which presumably underlies the progressively diminished memory function in older individuals. (After Dekaban and Sadowsky, 1978.) Memory and Aging Although outward appearance obviously changes with age, we tend to imagine that the brain is resistant to the ravages of time. Unfortunately, the evidence suggests that this optimistic view is unjustified. From early adulthood onward, the average weight of the normal human brain, as determined at autopsy, steadily decreases ( Figure 31.9 ). In elderly individuals, this effect can be observed with noninvasive imaging as a slight but nonetheless significant shrinkage of the brain. Counts of synapses in the cerebral cortex generally decrease in old age (although the number of neurons probably does not change very much), suggesting that it is mainly the connections between neurons (i.e., neuropil) that are lost as we grow old (consistent with the idea that the networks of connections that represent memories—i.e., the engrams—gradually deteriorate). These several observations accord with the difficulty many older people have in making associations (e.g., remembering names or the details of recent experiences) and with declining scores on tests of memory as a function of age. The normal loss of some memory function with age means that there is a large gray zone between individuals undergoing normal aging and patients suffering from age-related dementias such as Alzheimer's disease (see  Box D ).
• #35: Forgetting Some years ago, a poll showed that 84% of psychologists agreed with the statement that “everything we learn is permanently stored in the mind, although sometimes particular details are not accessible.” The 16% who thought otherwise should get the higher marks. Common sense indicates that, were it not for forgetting, our brains would be impossibly burdened with the welter of useless information that is briefly encoded in our immediate memory “buffer.” In fact, the human brain is very good at forgetting. In addition to the unreliable performance on tests such as the example in  Table 31.1 ,  Figure 31.5  shows that the memory of the appearance of a penny (an icon seen thousands of times since childhood) is uncertain at best, and that people gradually forget what they have seen over the years (TV shows, in this case). Clearly we forget things that have no importance, and unused memories deteriorate over time. The ability to forget unimportant information may be as critical for normal mentation as retaining information that is significant. One reason for this presumption is rare individuals who have difficulty with the normal erasure of information. Perhaps the best-known case is a subject studied over several decades by the Russian psychologist A. R. Luria, who referred to the subject simply as “S.” Luria's description of an early encounter gives some idea why S, then a newspaper reporter, was so interesting: I gave S a series of words, then numbers, then letters, reading them to him slowly or presenting them in written form. He read or listened attentively and then repeated the material exactly as it had been presented. I increased the number of elements in each series, giving him as many as thirty, fifty, or even seventy words or numbers, but this too, presented no problem for him. He did not need to commit any of the material to memory; if I gave him a series of words or numbers, which I read slowly and distinctly, he would listen attentively, sometimes ask me to stop and enunciate a word more clearly, or, if in doubt whether he had heard a word correctly, would ask me to repeat it. Usually during an experiment he would close his eyes or stare into space, fixing his gaze on one point; when the experiment was over, he would ask that we pause while he went over the material in his mind to see if he had retained it. Thereupon, without another moment's pause, he would reproduce the series that had been read to him. A. R. Luria (1987),  The Mind of a Mnemonist,  pp. 9–10 S's phenomenal memory, however, did not always serve him well. He had difficulty ridding his mind of the trivial information that he tended to focus on, sometimes to the point of incapacitation. As Luria put it: Thus, trying to understand a passage, to grasp the information it contains (which other people accomplish by singling out what is most important) became a tortuous procedure for S, a struggle against images that kept rising to the surface in his mind. Images, then, proved an obstacle as well as an aid to learning in that they prevented S from concentrating on what was essential. Moreover, since these images tended to jam together, producing still more images, he was carried so far adrift that he was forced to go back and rethink the entire passage. Consequently, a simple passage—a phrase, for that matter—would turn out to be a Sisyphean task. A. R. Luria (1987),  The Mind of a Mnemonist , p. 113 Although forgetting is a normal and apparently essential mental process, it can also be pathological, a condition called  amnesia . Some of the causes of amnesia are listed in  Table 31.2 . An inability to establish new memories is called  anterograde amnesia , whereas difficulty retrieving previously established memories is called  retrograde amnesia . Anterograde and retrograde amnesia are often present together, but can be dissociated under various circumstances. Amnesias following bilateral lesions of the temporal lobe and diencephalon have given particular insight into where and how at least some categories of memory are formed and stored (see next section and  Box C ). Why Do We Forget Explicit Memories? In this final part of Section 4, four major theories of why we forget explicit memories are discussed: reconstruction theory, encoding-specificity theory, interference theory, and defensive theory. Reconstruction Theory As just discussed, reconstruction theory states that forgetting occurs because the process of retrieval from the explicit-memory store introduces inaccuracies . Whenever we retrieve an explicit memory, we change its memory code to some extent by filling in gaps in the memory code with information obtained from other sources. The changed memory code will be resturned to the explicit-memory store until the next retrieval. Over time, the memory may become so transformed that it bears little resemblance to the information originally learned. A study that provided supporting evidence for reconstruction theory was performed by two cognitive psychologists, Ulric Neisser and Nicole Harsch (1992). These researchers decided to take advantage of a highly publicized tragedy, the explosion of the space shuttle Challenger, to study something called “flashbulb memories.” Flashbulb memories are defined as episodic memories of events that, because of their emotional intensity, are encoded in vivid detail that changes very little over time. The argument behind the concept of flashbulb memories was that, because very distressing events are attended to closely and, therefore, encoded in great detail, the events are "burned" into our memories, thereby resulting in an almost photographic memory code. Thus, many people claimed to remember exactly what they were doing and whom they were with when they first heard about the assassination of President Kennedy or the sinking of the Titanic . In order to test such claims, Neisser and Harsch, the day after the explosion of the Challenger in 1986, asked students in an introductory psychology course to fill out a questionnaire that asked them to describe the situation in which they first heard the news of the disaster: where they were, how they found out, who was with them, what they were doing, and what time it was. Since the event had occurred only 24 hours before they filled out the questionnaire, their episodic memories should have been very accurate. Three years later, Neisser and Harsch (1992) gave these students a new copy of the questionnaire and asked them to answer again the questions. They also were asked to rate how confident they were in the accuracy of their answers. The researchers compared the first set of answers to the second set for each student. They discovered that only 3 of the 44 students (7%) showed perfect recall. Thirty students (48%) recalled memories that contained varying amounts of accurate and inaccurate details. And 11 students (25%) recalled memories that were completely inaccurate. For example, one student provided the following answer 24 hours after the explosion: “I was in my religion class and some people walked in and started talking about [it]. I didn’t know any details except that it had exploded and the schoolteacher’s students had all been watching which I thought was so sad” (quoted in Ofshe & Watters, 1994, p. 39). Three years later, this same student provided a very different answer: “When I first heard about the explosion I was sitting in my freshman dorm room with my roommate and we were watching TV. It came on a news flash and we were both totally shocked. I was really upset and I went upstairs to talk to a friend of mine and then I called my parents” (p. 39). Neisser and Harsch found that over 90% of the students had memories that contained at least one major inaccuracy. What is even more surprising is that those with completely inaccurate memories were just as confident in the accuracy of their memories as were those with completely accurate memories! For example, the student quoted in this paragraph had complete confidence in the accuracy of her three-year-old memory — a memory that was false in all details. One semester after the second questionnaire was given, Neisser and Harsch (1992) asked the students to look at their answers to both questionnaires. The researchers expected that those who had misremembered the original event would realize their mistake after reading their answers to the first questionnaire and, after being reminded of what actually had occurred, then would remember the event accurately. But not one student did so. Although most were upset by the differences between the two sets of answers, their false memories did not change. They still felt certain that they remembered the event accurately, even after being shown incontrovertible evidence that this was not the case. Encoding-Specificity Theory Encoding-specificity theory states that forgetting is due to an inability to retrieve stored information when the retrieval cue is dissimilar to the form in which information was encoded for storage . As stated earlier, we probably always need some sort of retrieval cue to get information out of long-term memory. In order to serve adequately as a retrieval cue, the stimulus must have some similarity to the way in which we have stored that information in the long-term store. For example, a memory that is encoded in a tactile form (such as a memory for how an unseen bump on your back feels) is not likely to be retrieved by a cue that is in visual form (such as a picture of that same bump). The encoding-specificity principle states that the ability of a retrieval cue to facilitate (aid) the retrieval of a stored memory depends upon the degree to which it is similar to the memory code . Let’s say that there is a particular woman in your class whom you know by sight. In this case, your long-term memory for her is encoded in a visual form (when you think of her, you see an image of her that you have retrieved from your long-term store). Now, let’s say that you get assigned to a group with this person and she calls you in order to discuss the project. In this case, you may not know who she is because her voice, which is serving as the retrieval cue, is an auditory stimulus, not a visual one. In other words, because you have encoded your memory of her in a visual form, her voice is not an adequate retrieval cue for activating this memory. In a sense, you have “forgotten” who she is. There are more subtle examples of forgetting due to a mismatch between retrieval cues and memory codes. Let’s say that you have studied for a test primarily by memorizing the definitions of terms word-for-word. Rote memorization requires that you memorize the sounds of the words. Your memory codes for such definitions, therefore, would involve phonemic encoding. On a multiple-choice test, the retrieval cue is the correct answer (presented as one of several choices). If the correct answer is worded exactly as it was presented in the textbook, you should have no difficulty picking it because the form of the retrieval cue is identical to the form in which you encoded it. On the other hand, if the instructor gives an answer that is worded differently from the original definition — an answer that means the same thing — then the form of the retrieval cue is not similar enough to your memory code: the retrieval cue involves a semantic (meaning) code but you have a phonemic memory code. After the test, you may even complain angrily: “I don’t know where he got those questions! I know the answers weren’t anywhere in the book or in my notes!” In a sense, you are correct; but the problem was in the way you studied instead of in what you studied. If, during your studying, you had encoded information by forming more complex memory codes for the information (codes formed by using elaborative rehearsal), you would have been better able to choose the correct answers on the test. Thus, the encoding-specificity principle shows that how you encode information in working memory for transfer to long-term memory will affect your ability to retrieve this information later . In order to do your best on tests, you need to elaboratively rehearse information in working memory so that you can form complex semantic codes for your explicit memories. Not only do semantic codes allow for a more enduring long-term memory, they also allow a greater number of retrieval cues to activate any particular memory. Interference Theory Interference theory states that forgetting is due to an inability to distinguish several separate memories that are similar in some way . In other words, when separate items in memory share an important similarity (for example, several separate phone numbers or several separate names), we may begin to confuse the separate items and, hence, will have more difficulty extracting any particular item from the long-term store. There are two ways that interference can cause forgetting of explicit memories: (a) old information interferes with the memorizing of new information; (b) new information interferes with our memories for old information. For example, I often have trouble learning the names of my present students because I keep getting them confused with the names of former students (the older names make it hard for me to remember the new names) and I often have difficulty remembering the names of former students because I get them confused with the names of my present students (the new names make it hard for me to remember the older names). In general, interference is greatest when the competing items are similar in some way—the more similar, the greater the interference. There are limits to interference. In fact, we often find that learning similar information in two different courses can help you to better remember the information. Such facilitation of learning is probably much more common than interference. Why isn’t interference occurring in this situation? Well, it is, but it depends on what we are testing for. We probably would see interference if I asked you in which of these two courses you had learned a particular item of information. But of course, no one asks you such questions on a test. Your teachers simply want you to learn the material, not also remember in which class you learned it. Thus, you should remember that interference studies are somewhat contrived and artificial: they involve rote memorization of very similar and simple material.  In general, it is probably safe to say that interference is an important cause of forgetting only when we learn similar items of information that we need to keep separate in our minds for later retrieval . For example, parking in the lot outside of school each and every day can cause problems when you are trying to remember where you parked today: you probably keep getting this confused with where you parked during the past week. Nevertheless, parking in the same lot each day also facilitates your learning of other material. For example, you probably have learned a great deal about where you are most likely to find an open space at a particular time. Defensive Theory Defensive theory states that forgetting is due to the removal of mental content from consciousness because it causes too much anxiety when attention is focused on it . In other words, this theory states that a person may transform an explicit memory into an implicit one when the information contained in the memory distresses him or her. According to this theory, people do this because, if they no longer are aware of the memory, then their distress disappears. For example, if you had seen one of your parents temporarily become very ill when you were a young child, your memory for this event (even after the parent had recovered) may have caused you a great deal of anxiety. If you were not able to cope at this time with your memory of the event (for example, if you were too young to be able to say to yourself something like, “well, that was upsetting, but now it is over and everything is alright”), then you would have felt severe anxiety whenever you remembered it. At this point, according to defensive theory, you may have transformed the explicit memory into an implicit one by using what I will call “defensive forgetting.” If you do not now remember certain traumatic events from your childhood that you know happened (because others have told you they did), defensive theory states that defensive forgetting may be the cause. Although the concept of repression is widely accepted among therapists and the general public, the evidence for it is not very good. There is one major problem with most research on repression: because repression is thought to work unconsciously — and, thus, has no unambiguous (clear) effects on our conscious mental events and behaviors — it is difficult to find examples of this defense mechanism that cannot be explained in other ways . One needs controlled research to rule out the various possibilities. As I stated earlier, the evidence supporting repression consists mostly of clinical case studies that use some version of recovered-memory therapy. Case studies, however, involve the use of uncontrolled research situations. The results of few (if any) laboratory studies using adequate controls give support to the concept of repression. As Holmes (1990) concluded in his review of decades of research on repression: “despite over sixty years of research involving numerous approaches by many thoughtful and clever investigators, at the present time there is no controlled laboratory evidence supporting the concept of repression” (p. 96). He stated that clinicians often dismiss these studies and instead focus on evidence involving case studies. Holmes, however, correctly argued that case studies do not provide adequate evidence for any claim, including claims about the existence of repression. He suggested in jest that anyone using the concept of repression should also provide the following cautionary label: “Warning. The concept of repression has not been validated with experimental research and its use may be hazardous to the accurate interpretation of clinical behavior” (p. 97). Because of this lack of controlled experimental evidence for repression, cognitive psychologists have questioned whether or not repression actually occurs. They are especially concerned about those "memories" that return after weeks, months, or years of therapeutic work. They base their criticism on the evidence for reconstruction theory showing that, when we retrieve a long-term memory, we reconstruct it by combining the fragments of information contained in the activated engram with the large amount of information contained in activated schemas. In this view, each reconstructed memory is always a combination of fact and fiction. In other words, according to reconstruction theory, all of our memories are inaccurate to various degrees . When a person is strongly encouraged to retrieve memories of events that may not have occurred (as they are in recovered-memory therapy), it seems possible that memory reconstruction could lead to predominantly false memories — memories that include true but irrelevant details surrounded by a fictional story. For example, clients receiving recovered-memory therapy often are encouraged to visualize traumatic childhood events that they suspect may have occurred. It is plausible that such a technique could cause the person to incorporate imagined events into the reconstructed “memory.” Ofshe and Watters (1994) suggested that the process could occur something like this: Picture an elephant. Imagine an apple. Now spend a moment visualizing an image of being sexually assaulted by one of your parents. It is an often distressing trick of the mind that it will create any event regardless of our desire to visualize that event. What separates an imagined image from a memory image is not a simple matter, for even imagined events are themselves largely built from memory. Our ability to imagine an elephant would be impossible if we didn’t have a memory of having seen an elephant or a picture of the creature. Similarly, our ability to ... imagine a sexual assault by a parent would also come from an amalgam of memories. To create this image we might use recollections of our parents’ physical appearances and of ourselves as children. We might place the scene in the memory of our childhood room. To create the action of the scene, we might use memories of other people’s descriptions of sexual assaults or of abuse scenes depicted in books or movie dramas. In the end, all the pieces of the imagined event would have something of the weight of memory. (p. 107) These imagined events would be combined with bits of remembered events from childhood (for example, the night one of your parents came into your bedroom to open your windows after the air conditioner broke) to produce a remembrance that contains a small slice of truth and a large amount of fiction — a remembrance that accords with the therapist’s suggestion that you have repressed memories of traumatic events.