Visual science: All in one
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This document discusses various topics related to visual science including: 1. Spatial vision is defined as the perception of objects with different luminance profiles in the visual field. Key attributes of spatial targets are frequency, contrast, phase, and orientation. 2. Fourier transformations can be used to construct complex spatial stimuli by adding sine waves of proper frequency, contrast, phase, and orientation. 3. The spatial modulation transfer function describes how well a lens or lens system transfers spatial information and how image quality is degraded by aberrations and reduced contrast at higher frequencies. 4. Temporal vision involves detecting time-related changes in spatial target properties and is described using concepts like temporal frequency, depth of modulation, and critical
Visual science: All in one
- 2. Spatial Vision • Objects with different luminance profile in visual field are considered as spatial target and the perception is called spatial vision.
- 3. Four attributes we should define while specifying a detail on as spatial target: i) Frequency: - Frequency simply means how close the spatial targets lie or how close are two light or dark bands. - Frequency is measured in cycles/degree. - More cycles in a degree means more frequency. ii) Contrast: 𝑪𝒐𝒏𝒕𝒓𝒂𝒔𝒕 = 𝑰𝒎𝒂𝒙 − 𝑰𝒎𝒊𝒏 𝑰𝒎𝒂𝒙 + 𝑰𝒎𝒊𝒏 = ∆𝑰/𝑰𝒂𝒗𝒈
- 4. iii) Phase: - Position of a sine wave grating with respect to another grating is known as phase. - If two grating are 180 degree out of phase two light/dark grating overlap with each other. iv) Orientation: - Orientation refers to the angle made by grating with respect to a reference, such as horizontal.
- 5. Fourier Wave transformation: • Sine waves of proper frequency, contrast, phase and orientation can be used to construct other more complex spatial stimuli. • Add one trough/dark with another trough/dark. • Keep on adding the wave and the last final wave obtained will be a square wave with 100 % blackness or whiteness.
- 10. Spatial Modulation Transfer Function: • It tells us about how perfectly a lens or lens system transfers information about the object. • No lens is perfect, there is always degradation in quality of image formed. • There is reduction in contrast and image clarity is affected even more by inherent aberration within lens. • This problem remains unnoticed most of the time at lower spatial frequency than at higher spatial frequency.
- 11. Human Contrast Sensitivity Function • Consider three spatial frequency gratings at 100 % contrast. • One with lower, other with medium and last one with higher spatial frequency. • Keep on decreasing the contrast of all three grating until all grating will start to fade slowly. • Keep on decreasing the contrast until the last just one that is visible remains. • Typical human contrast sensitivity can be plotted in a graph. It is found that human contrast sensitivity function peaks at spatial frequency of 4cycles/degree.
- 12. CSF High Frequency Cut Off • CSF shows reduction in sensitivity at high spatial frequency. • As spatial grating frequency is kept on increasing, a point is reached where grating can no longer resolved at even at 100% contrast. • For normal adult, its value is 60 cycle/degree. • By measuring Visual acuity of a person we are measuring the high frequency cut-off value for him/her High frequency cut-off value is limited by inherent optical aberration of human lens system and photoreceptor packing Density
- 13. Low Frequency Drop Off CSF • Receptive field of typical ganglion cell manifests centre- surround region that respond in opposing fashion to illumination. • Light falling on centre causes excitation, while light falling on surrounding causes inhibition. • Cell will be optimally activated when bright bar falls on centre of its receptive field and dark bar on its surrounding. • when Both centre and surround is activated by single iso- luminance grating the excitation and inhibition occurs simultaneously; thus reducing the visibility of a target. • Though the target is seen it is not perceived as the part of a single grating system.
- 14. Mach bands: • As seen in the grating where there is abrupt change in luminance profile from dark to light or vice versa. • Human eye is capable of separating the junction between two luminance profile. • But in practical life situation transition between dark and light region is gradual. • Human eye will only perceive drastic luminance change zone but can’t perceive the zone with gradual change • This invisible zone of gradual luminance change is called Mach Bands.
- 15. Temporal vision • Insert time into a spatial target and we get a temporal target. • Temporal target is the target with change in luminance profile over time. • Even here an ideal target for describing temporal vision is a spatial grating modulated along with time. • Ability of our visual system to detect time related change in the property of a spatial target is called temporal vision • That blinking rate of the spatial target is temporal frequency.
- 16. Temporal Frequency: • Flickering light bulb is an example of temporally modulated stimulus at high frequency. • Hour hand of a clock moves at very slow temporal frequency. which makes it impossible to for our visual system to detect any temporal changes. • Temporal target can also be explained with wave function. • For light off situation its trough and for light on situation its crest • Presentation of spatial target can be represented by change in sinusoidal wave over time.
- 17. Depth of Modulation • Target refers to the amplitude of change in luminance over time. • 𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑀𝑜𝑑𝑢𝑙𝑎𝑡𝑖𝑜𝑛 = 𝐴 𝐼𝑎𝑣𝑔 𝑥100 • A =amplitude of modulation • lave = time-averaged luminance
- 18. Temporal Modulation Transfer Functions: • Consider a light flickering at low frequency. Increase the frequency of the light flickering until no flickering is seen. • Change the depth of the modulation maintaining the same temporal frequency; the light will be seen flickering again. • Keep on increasing temporal frequency such that further depth modulation doesn’t cause any flickering. • The resulting temporal frequency is called High Frequency Cut-off value. • Repeat the process but in reverse order. • Decrease the temporal frequency and depth of modulation until light is seen steady or change in illumination over time is not perceived. • This low frequency where light is seen no more flickering is called Low Frequency Drop-off. Hence, for every percentage of modulation it has its own high frequency cut-off and low frequency drop-off value.
- 19. Critical Flicker Fusion Frequency (CFF): • Temporal Frequency where flicker can no longer be resolved is called Critcal Flicker Frequency. • High temporal resolution limit of the visual system for a given depth of modulation. • Typically given in Hertz (Hz) i.e. 1 Hz = one cycle/second.
- 20. Broca–Sulzer Effect: • Suprathreshold flashes of light with a duration on the order of 50 to 100 ms appear brighter than stimuli of either shorter or longer durations, a phenomena referred to as the Broca–Sulzer effect.
- 21. Brucke-Bartley Effect: • Light presented at approximately 10 Hz, which is seen as flickering, appears brighter than a steady light of the same average luminance. • Brücke–Bartley effect is a manifestation of the Broca–Sulzer effect.
- 22. Talbot-Plateau Law: • Brightness of a temporally modulated stimulus, when fused, is equal to the brightness of a steady light with the same time-averaged luminance. • The temporally modulated light must be presented at a rate beyond the CFF.
- 23. Color Measurement System • Three color making attributes are Hue, Brightness and Saturation. a) Hue: Hue is that perception which is most closely associated with wavelength. b) Saturation: Desaturated color appears as though it has been mixed with white. - A monochromatic stimulus, which by definition has no white light added to it, is said to have a colorimetric purity of 1. Colorimetric purity, a physical property of the stimulus, is given, by the following formula: Where 𝑝 = 𝐿ʎ 𝐿ʎ + 𝐿w p=colorimetric purity Lʎ= luminance of the monochromatic light Lw =luminance of the white light that is combined with the monochromatic light c) Brightness: Brightness sensation closely follows the photopic luminance function
- 24. Bezold–Brücke phenomenon: • Most monochromatic stimuli slightly change hue as their intensity is adjusted. • Consider a greenish yellow test wavelength of 550 nm. As the intensity of this wavelength is increased, it appears to be of a longer wavelength (i.e., it appears more yellowish). • To maintain the initial hue appearance (greenish yellow), it is necessary to reduce its wavelength. • This is indicated in Figure by the line that starts at 550 nm and tilts toward shorter wavelengths as the intensity increases. • These wavelengths, 478, 503,and 578 nm, are referred to as invariant wavelengths or invariant points. • The hues associated with these wavelengths—blue at 478 nm, green at 503 nm, and yellow at 578 nm—are called unique hues.
- 25. Color constancy: • Color constancy refers to the approximately constant color appearance of objects as lighting conditions change. • Color constancy assists us in identifying objects as lighting conditions vary. • Red apple appears approximately the same color both indoors and outdoors and at various times of the day.
- 26. Physiological Basis of Color Vision: • There are two main theories that explains color vision: i) Trichromatic Theory ii) Color opponent theory
- 27. Trichromatic Theory (Young-Helmholtz theory) • According to this theory there are three cones in Retina that are responsible for color vision. Namely red, green and blue cone. • Mixture occurs by differential excitation and absorption of various wavelength which are derivative of these primary colors. • Once a quantum of light is absorbed, all information regarding its wavelength is lost, a principle referred to as univariance. • Helmholtz used color-matching experiments where participants would alter the amounts of three different wavelengths of light to match a test color. • Participants could not match the colors if they used only two wavelengths but could match any color in the spectrum if they used three. • The theory became known as the Young-Helmholtz theory of color vision.
- 28. Color opponent theory (Hering’s Opponent-Colors Theory) • The opponent-process theory states that the cone photoreceptors are linked together to form three opposing color pairs: blue/yellow, red/green, and black/white. • Activation of one member of the pair inhibits activity in the other. • A color perception is never described as reddish-green or yellowish-blue. • Besides the cones, which detect light entering the eye, the biological basis of the opponent theory involves two other types of cells: bipolar cells, and ganglion cells.
- 30. Color measurement System: • This measurement is necessary because we have to accurately connect an external light or surface with an internal color sensation. a) Munsell Color measurement system - Munsell color system is a color space that specifies colors based on three properties of color: hue, value (lightness), and chroma (color purity). - Munsell divided into five principal hues: Red, Yellow, Green, Blue, and Purple, along with 5 intermediate hues.
- 31. Maxwell Color Triangle • A color triangle is an arrangement of colors within a triangle, based on the additive combination of three primary colors at its corners. • Scottish physicist James Clerk Maxwell (1831-1879), He demonstrated that most colors could be created by combinations of three "primary" colors.
- 32. CIE color Space • Commission Internationale de l'éclairage or International Commission on Illumination. • CIE 1931 color spaces were the first defined quantitative links between distributions of wavelengths in the electromagnetic visible spectrum, and physiologically perceived colors in human color vision. • Essential tools for color management, important when dealing with color inks, illuminated displays, and recording devices such as digital cameras. • Relative amounts of primaries are denoted by x,y, & z. So their sum, x+y+z=1. The tristimulus values are given by uppercase letters, whereas the chromaticity coordinates are given in lowercase letters.
- 33. • Two wavelengths can be mixed at any proportion. Amount of proportion of two wavelengths can be represented with coordinates joining those two wavelengths • Length of line connecting white and mixture coordinate point and white and perimeter hue point gives the value of calorimetric purity (in CIE Excitation Purity). Those two wavelengths which can be mixed at equal proportions to give a white color are called Complementary Colors.
- 34. Motion Perception • Perception of object or target moving is know as motion perception. • Motion perception is due to change in spatial distribution of light over time. • Motion can be either real motion where target or subject actually moves or it could be illusory motion where spatial targets are doesn’t move but we perceive motion. • Middle temporal area (MT or V5) and medial superior temporal area (MST or V5a) involved in motion perception Real Motion: -Real motion occurs when target actually moves. When target moves the spatial distribution of target image changes across retina.
- 35. Apparent motion/Illusory motion: a) Stroboscopic movement b) Induced movement c) Autokinetic movement d) Movement after-effect
- 36. Stroboscopic movement: Movement between two lights, when they are switched on and off alternately. The nature of movement between the flashing lights depend upon both time and distance between them. • Simultaneous Perception: if the time interval between the two flashes is less than 30 msec, they are perceived to flash on and off simultaneously. • Partial perception- as the interval is increased to about 30 msec, partial movement is perceived. • Optimal movement- at an interval of about 60 msec, the light appears to move continuously. Looks like real movement. Film frame projection rate. • Phi-movement- at an interval of around 60-200 msec, type of movement perceived is objectless. Although movement appears to occur but it is difficult to perceive the object
- 39. Induced movement: - Light surrounded by larger objects move so that the light appears to move in opposite direction. E.g. perception of moon racing through the clouds while it is the clouds that are moving. Autokinetic movement/effect - When a small stationary spot of light is observed in a dark room, the spot of light appears to move. - It can be demonstrated by looking at a lit cigarette across a dark room. Movement after-effect - Moving stripes viewed prior to light. - Light appears to move in opposite direction to the motion of stripes. - Also called waterfall illusions- if one stares at a waterfall about 30-60 secs and looks away towards something else, he will see the scenery move upwards
- 40. Autokinetic Movement after effect
- 41. Order of Motion 1st-order motion : First order motion, which is what we encounter in our daily lives, can be easily detected by finding the peak in the Fourier energy distribution. - This is motion due to luminance change 2nd-order motion - Second-order motion has been defined as motion in which the moving contour is defined by contrast, texture, flicker or some other quality
- 42. • The smallest percentage of coherence level that gives perception of motion is called Coherence threshold. • The minimum distance dots have to move in given direction to elicit the perception of motion is called Minimum Displacement threshold. • The maximum separation allowed for still to perceive motion is called Maximum displacement threshold. This temporary suppression of movement of visual world during saccades or pusuits is called saccadic suppression.
- 43. Threshold and Signal Detection Theory • Any stimulus that can be detected by our visual system for 50% time of the presentation is called the threshold of the stimulus. • Range of stimulus intensities, from dim to intense, and the percentage of stimuli detected is plotted as a function of stimulus intensity to produce a frequency of seeing (FOS) curve, also referred to as a psychometric function. For ideal observer
- 44. Determination of threshold Method of Ascending Limits ii) Method of Descending Limits iii) Staircase Method iv) Method of Constant Stimuli v) Method of Adjustment vi) Forced Choice Method
- 45. Method of Ascending Limits: - Stimulus is initially presented below threshold. - The stimulus intensity is increased systematically until the observer reports that it is visible. - Several trials may be performed and observations are averaged. Eg: Dark Adaptometry
- 46. Method of Descending Limits: • A trial commences with a clearly visible stimulus (i.e., the stimulus is above threshold) • The visibility is decreased systematically until it can no longer be seen. • The method of descending limits may be contaminated by observer anticipation. E.g.: Visual Acuity test
- 47. Staircase Method: • Combination of ascending and descending limits • Increase visibility of subthreshold stimulus until observer reports seeing the stimulus. • Reverse visibility of the stimulus is reduced until the observer reports that it cannot be detected. • Threshold is taken to be the stimulus intensity at one of the reversals, for example, the fourth reversal. Eg: Full threshold Strategy of Automatic Visual field
- 48. Method of Constant Stimuli • Stimulus visibility is varied randomly from presentation to presentation. • Because the observer is typically asked whether or not he or she sees the stimulus. • Sometimes referred to a “yes–no” procedure • A FOS curve is plotted and 50% visibility is taken as the threshold. e.g. Supra threshold Strategy in Visual field
- 49. Method of Adjustment • Subject adjusts the stimulus intensity until it is barely visible (or invisible). • Anticipation and variations in the observer’s threshold criterion
- 50. Forced Choice Method • Observer is forced to choose between several alternative choices, one of which contains the stimulus. • If the experiment forces the observer to choose between two alternatives, it is referred to as a two alternative forced choice (2AFC) experiment • AFC for 4 alternatives as in figure below. e.g. Forced choice preferential test
- 51. SIGNAL DETECTION THEORY • The theory assumes that within the visual system there is a randomly fluctuating level of background neural activity—the so-called noise. • The observer’s task is to differentiate the signal and noise combination from the background noise alone. • Influenced by a number of factors, including decision criteria, attention, motivation, and internal neural noise. Eg; Muliple optotypes target letter visibility is affectd by nearby letters (crowding Phenomena)
- 53. Effect of Observer Criterion • Hit • Miss • False Positive • True Negative (correct Rejects) Types of observer/ Criteria: a) Strict Criteria: Responds only when complete confirmation of stimulus. More False Negatives. b) Lax criteria: Responds even when there is not stimulus (eg. Beep sound in AVF). More False Positives. Trigger Happy Patients. False Negative
- 54. Receiver Operating Characteristic Curves • A receiver operating characteristic (ROC) curve, which shows the probability of a hit as a function of the probability of a false positive. • Allows us to predict the effect of the observer criteria for a given detectability (dʹ). L: Lax S: Strict M: average
- 56. WEBER’S LAW • Just noticeable difference (JND) or Difference Limen (DL): 10 kg--------------11 kg X 10 kg---------------12 kg √ (JND: 2 kg) 50 kg --------------- ??? kg Ratio: ∆w/w = 2/10 = 0.2 For Particular JND ratio is always same e.g. Ratio = ∆w/w 0.2 = ∆w/ 50 ∆w = 10 kg Hence next weight is: (50+10= 60kg)
- 57. • This process—sensitivity regulation—results in a constant contrast threshold regardless of the background brightness. • For scotopic vision, this contrast threshold is 0.14 (or 14%); for photopic vision it is 0.015 (or 1.5%).