2. Sensation vs. Perception
• A somewhat artificial distinction
• Sensation: Analysis
– Extraction of basic perceptual features
• Perception: Synthesis
– Identifying meaningful units
• Early vs. Late stages in the processing of
perceptual information
3. The parts without the Whole
• When sensation seems to happen without
perception: Agnosia
• Agnosia = “without knowledge”
• Seeing the parts but not the whole object
• Prosopagnosia: The man who mistook his
wife for a hat
4. The Problem of Perception:
Perceiving 3D objects from a 2D Stimulus
I) Four “Information Processing” approaches:
• Template matching
• Feature matching
• Prototype matching
• Structural descriptions
II) A connectionist approach
III) The “ecological optics” approach
5. Template Matching
• Objects represented as 2-D arrays of pixels
• Retinal image matched to the template
• Viewer-centered
• Problems:
– Orientation-dependent
– Inefficient?
• 2 Stages: Alignment, then Matching
6. Feature Analysis
• Objects represented as sets of features
• Retinal image used to extract features
• Object-centered
• Example: Pandemonium (Selfridge, 1959)
– Model of word recognition
– Features -> Letters -> words
– Heirarchical and bottom-up
• Neurological “feature detectors”
7. Hubel & Wiesel (1959, 1963)
• Specific cells in cat and monkey visual
cortex responded to specific features
– Simple cells
– Complex cells
– Hyper-complex cells
8. Feature Analysis: Advantages
• Some correspondence to neurology (at early
levels)
• Economical: only 1 representation stored
for each object
9. Feature Analysis: Disadvantages
• Not every instance of the pattern has all the
features (see prototype theories)
• Does not take into account how the features
are put together (see structural description
theories)
• Some features may be obscured from
different points of view (see structural
description theories again)
10. Prototype Matching Theories
• Prototype = a typical, abstract example
• Objects represented as prototypes
• Retinal image used to extract features
• Object recognition is a function of
similarity to the prototype
11. Prototypes: Advantages
• Accounts for the intuition that some
features matter more than others
• Is more flexible – recognition can proceed
even if some features are obscured
• Accounts for “prototype effects” – objects
more similar to the prototype are easier to
recognize
12. Example of Prototype Effects
• Solso & McCarthy (1981)
• Identikit faces
• Study faces similar to a “prototype”
14. Solso & McCarthy Results
• Recognition test
• Recognition confidence was a function of
number of features shared with prototype
• Prototype face was most confidently
“recognized” even though it was not studied
• (Note: Exemplar theories can also predict
this result)
15. Solso & McCarthy Results
Pattern of Results (not actual data)
0% 25% 50% 75% 100%
Features Shared with Prototype
Confidence
that
Face
was
"Old"
17. Structural Description Theories
• Objects represented as configurations of
parts (features plus relations among
features)
• Retinal image used to extract parts
• Object-centered
• Example: Biederman’s Structural
Description Theory
18. Structural Description Theory
(Biederman)
• Objects are represented as arrangements of
parts
• The parts are basic geometrical shapes or
“Geons”
• Object-centered
• Evidence: degraded line drawings
19. Structural Description Theory
• Advantages
– Recognizes the importance of the arrangement
of the parts
– Parsimonious: Small set of primitive shapes
• Disadvantages
– Structure is not always key to recognition:
Peach vs. Nectarine
– Which geons? (simplicity vs. explanatory
adequacy)
20. Another Problem…
• All of these theories are basically “bottom-
up”
• None can account very well for context
effects (top-down)
c
21. Top-down and Bottom-up
Processing
• Bottom-up: Stimulus driven; the default
• Top-down: Context-driven or expectation-
driven. Examples:
– Word superiority effect (see Coglab)
– McGurk Effect (
http://www.media.uio.no/personer/arntm/McGurk_english.html)
22. The Interactive Activation Model
• A connectionist model of word recognition
• Incorporates both top-down processing
(forward connections) and bottom-up
processing (backward connections)
• The nodes sum activation
• Connections can be excitatory or inhibitory
• Run the Model: http://www.socsci.kun.nl/~heuven/jiam/
23. Gibson’s Ecological Optics:
an alternative view
• Constructivist models vs. direct perception
• Constructivist models
– Stimulus information underdetermines
perceptual experience (e.g., depth perception)
– Rules (unconscious inferences) must be applied
to the stimulus information to achieve
perception
– Top-down processes compensate for the
poverty of the stimulus
24. Direct Perception
• All the information is in the stimulus
• Most stimuli are not ambiguous
• Motion provides information
• Invariants – properties of the stimulus that
are invariant across changes in viewpoints
and can be directly perceived
• Entirely stimulus-driven (bottom-up)
25. Invariants
• Center of expansion – always is the point
you are moving towards
• Texture gradients – always become less
course as distance increases
26. Evidence that Motion is
Important:
• Center of expansion can induce perception
of motion (starfield screen-savers)
• Human figures can be recognized from
moving points of light
27. Problems for Direct Perception
• There are top-down effects on perception
• Depth perception is possible even when
motionless
• Depth can even be extracted from “random
dot” stereograms without motion
– Stereogram of the week: http://www.magiceye
.com/3dfun/stwkdisp.shtml
28. Integrating Visual Perception
Across Space and Time
• How do we integrate visual information
across space and time?
• Not as well as you might think
• Across Space: Impossible figures
• Across Time: Change blindness
31. Change Blindness
• Integrating across time: saccades
• Change blindness
http://www.usd.edu/psyc301/ChangeBlindness.htm
• Why did our visual system evolve this way?
32. Perceptual Illusions
• Systematic distortions of reality caused by
the way our perceptual system works
• Questions to ask as you view them:
– What does this phenomenon tell me about the
mechanisms at work in perception?
– Does this illusion result from top-down or
bottom-up processes?
– Is there a formal model that could explain this
perceptual illusion?
33. Perceptual Illusions: web sites
• http://www.rci.rutgers.edu/~cfs/305_html/Gestalt/Illusions.html
• http://www.cfar.umd.edu/users/pless/illusions.html
• http://www.psych.utoronto.ca/~reingold/courses/resources/cogillusion.
html
