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Cognitive Processes PSY 334. Chapter 2 – Perception. Object Recognition. Two stages: Early phase – shapes and objects are extracted from background. Later phase – shapes and objects are categorized, recognized, named. Disruptions of Perception.
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Cognitive ProcessesPSY 334 Chapter 2 – Perception
Object Recognition • Two stages: • Early phase – shapes and objects are extracted from background. • Later phase – shapes and objects are categorized, recognized, named.
Disruptions of Perception • Visual agnosias – impairment of ability to recognize objects. • Demonstrate that shape extraction and shape recognition are separate processes. • Apperceptive agnosia (lateral) – problems with early processing (shape extraction). • Associative agnosia(bilateral) – problems with later processing (recognition). • Prosopagnosia – visual agnosia for faces.
Tests for Apperceptive Agnosia Some patients would have trouble drawing this chair due to the missing contours. Some patients would have trouble recognizing a chair from this perspective.
Tests for Associative Agnosia The subject can copy the anchor accurately (as shown) but then cannot tell you what it is.
Early Visual Processing • Parts of the eye • Two kinds of photoreceptors: • Rods respond to motion, light & dark • Cones respond to color, shape, detail • Fovea is the area of the retina with highest resolution – best for seeing detail. • We move our eyes so light hits the fovea.
Later Visual Processing • Neural pathways from the eyes to the visual cortex split at the optic chiasm. • Info from the left visual field goes to the right hemisphere. • Info from the right visual field goes to the left hemisphere. • Two pathways from the visual cortex: • “Where” pathway • “What” pathway
Information Coding • On-off cells in LGN feed into edge and bar detectors in the visual cortex. • Edge detectors – respond positively to light on one side of a line, negatively on the other side of the line. • Bar detectors – responds maximally to a bar of light covering its center.
Feature Maps • In addition to edges, lines, bars, other information is extracted from the visual signal: • Color • Motion • These aspects, called “features,” are represented in feature maps located in different areas of the brain.
Depth Perception • Our eyes turn a three-dimensional world into a two-dimensional image on the retina. • Our cortex turns that two-dimensional image back into three-dimensions (depth). • Cues are used to infer distance. • Cues must be learned through experience. • Depth cues in art: http://psych.hanover.edu/KRANTZ/art/cues.html
Optic flow Nearer things move faster, farther things move slower
Marr • Depth cues (texture gradient, stereopsis, motion parallax) – where are edges in space? • How are visual cues combined to form an image with depth? • 2-1/2 D sketch – identifies where visual features are in relation to observer. • 3-D model – refers to the representation of the objects in a scene.
Pattern Recognition • Classification and recognition of objects occurs through processes of pattern recognition. • Bottom-up processes – feature detection • Top-down processes -- conceptually driven processing
Top-Down Processing Why do we see an H in the first word but an A in the second word?
Gestalt Priniciples • Wertheimer, Koffka, Kohler. • Form perception – segregation of a display into objects and background. • Principles of perceptual organization allow us to see “wholes” (gestalts) formed of parts. • We do not recognize objects by identifying individual features.
Five Principles • Proximity • Similarity • Good continuation • Closure • Common fate • Elements that move together group together. • These will be on the midterm.
Examples (Fig 2-13) proximity similarity good continuation closure
Examples • Gestalt principles of organization • http://psych.hanover.edu/Krantz/sen_tut.html • Illusory contours: http://psych.hanover.edu/JavaTest/Media/Chapter5/MedFig.IllusoryContour.html • Reversible figures • http://www.psy.ritsumei.ac.jp/~akitaoka/reversiblee.html • Apparent motion demos: http://psy.ucsd.edu/~sanstis/SACamov.html http://www.michaelbach.de/ot/mot_biomot/index.html http://www.lifesci.sussex.ac.uk/home/George_Mather/BM_ECVP_2006.htm
Law of Pragnanz • Of all the possible interpretations, we will select the one that yields the simplest or most stable form. • Simple, symmetrical forms are seen more easily. • In compound letters, the larger figure dominates the smaller ones.
Law of Pragnanz People are more likely to see (b) and (c) not (d) or (e) in figure (a)
Visual Illusions • Depend on experience. • Influenced by culture. • Illustrate normal perceptual processes. • These are not errors but rather failures of perception in unusual situations. • Try some yourself: • http://www.michaelbach.de/ot/
Visual Pattern Recognition • Bottom-up approaches: • Template-matching • Feature analysis • Recognition by components
Template-Matching • A retinal image of an object is compared directly to stored patterns (templates). • The object is recognized as the template that gives the best match. • Used by computers to recognize patterns. • Evidence shows human recognition is more flexible than template-matching: • Size, place, orientation, shape, blurred or broken (ambiguous or degraded items easily recognized by people.
Feature Analysis • Stimuli are combinations of elemental features. • Features are recognized and combined. • Features are like output of edge detectors. • Features are simpler, so problems of orientation, size, etc., can be solved. • Relationships among features are specified to define the pattern.
Evidence for Feature Analysis • Confusions – people make more errors when letters presented at brief intervals contain similar features: • G misclassified: as C (21), as O (6), as B (1), as 9 (1) • When a retinal image is held constant, the parts of the object disappear: • Whole features disappear. • The remaining parts form new patterns.
Object Recognition • Biederman’s recognition-by-components: • Parts of the larger object are recognized as subobjects. • Subobjects are categorized into types of geons – geometric ions. • The larger object is recognized as a pattern formed by combining geons. • Only edges are needed to recognize geons.
Tests of Biederman’s Theory • Object recognition should be mediated by recognition of object components. • Two types of degraded figures presented for brief intervals: • Components (geons) missing • Line segments missing • At fast intervals (65-100 ms) subjects could not recognize components when segments were missing.
Face Recognition • Prosopagnosia – inability to recognize familiar faces. • Are faces special? • Thatcher effect • Damage to fusiform gyrus causes prosopagnosia. • The area may also be used for fine-grained distinctions needed to recognize faces but also other objects. • Bird, car & greeble experts all use it.
Identification of Faces and Members of CategoriesProsopagnosia The Fusiform Face Area: http://www.psy.vanderbilt.edu/faculty/gauthier/picts/mona_lisa.jpg
Thatcher Illusion (Cont.) Why did it look more normal when viewed upside down?
Greebles & Faces Figure 4.24 (a) Greeble stimuli used by Gauthier. Participants were trained to name each different Greeble. (b) Brain responses to Greebles and faces before and after Greeble training. (a: From Figure 1a, p. 569, from Gauthier, I., Tarr, M. J., Anderson, A. W., Skudlarski, P. L., & Gore, J. C. (1999). Activation of the middle fusiform “face area” increases with experience in recognizing novel objects. Nature Neuroscience, 2, 568-573.)
Speech Recognition • The physical speech signal is not broken up into parts that correspond to recognizable units of speech. • Undiminished sound energy at word boundaries – gaps are illusory. • Cessation of speech energy in the middle of words. • Word boundaries cannot be heard in an unfamiliar language.
Phoneme Perception • No one-to-one letter-to-sound correspondence. • Speech is continuous – phonemes are not discrete (separate) but run together. • Speakers vary in how they produce the same phoneme. • Coarticulation – phonemes overlap. • The sound produced depends on the sound immediately preceding it.
Feature Analysis of Speech • Features of phonemes appear to be: • Consonantal feature (consonant vs vowel). • Voicing – do vocal cords vibrate or not. • Place of articulation – where the vocal track is constricted (where is tongue placed). • The phoneme heard by listeners changes as you vary these features. • Sounds with similar features are confused.