Cognitive Processes PSY 334 Chapter 2 Perception June
- Slides: 18
Cognitive Processes PSY 334 Chapter 2 – Perception June 30, 2003
Visual Perception o Distal stimulus -- tree o Proximal stimulus – image of tree on retina o Percept – interpretation of proximal stimulus as a tree n Size and color constancy
Information Coding o On-off cells in LGN feed into edge and bar detectors in the visual cortex. o Edge detectors – respond positively to light on one side of a line, negatively on the other side of the line. o Bar detectors – responds maximally to a bar of light covering its center.
Marr o Depth cues (texture gradient, stereopsis) – where are edges in space? o How are visual cues combined to form an image with depth? n n 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 o Classification and recognition occurs through processes of pattern recognition. o Bottom-up processes – feature detection o Top-down processes -- conceptually driven processing
Object Recognition o Two stages: n n Early phase – shapes and objects are extracted from background. Later phase – shapes and objects are categorized, recognized, named.
Disruptions of Perception o Visual agnosias – impairment of ability to recognize objects. n Demonstrate that shape extraction and shape recognition are separate processes. o Apperceptive agnosia (lateral) – problems with early processing (shape extraction). o Associative agnosia (bilateral) – problems with later processing (recognition). o Prosopagnosia – visual agnosia for faces.
Gestalt Priniciples o Wertheimer, Koffka, Kohler. o Form perception – segregation of a display into objects and background. o Principles of perceptual organization allow us to see “wholes” (gestalts) formed of parts. n We do not recognize objects by identifying individual features.
Five Principles o Proximity o Similarity o Good continuation o Closure o Common fate n Elements that move together group together.
Examples • Gestalt principles of organization • Reversible figures
Law of Pragnanz o Of all the possible interpretations, we will select the one that yields the simplest or most stable form. o Simple, symmetrical forms are seen more easily. o In compound letters, the larger figure dominates the smaller ones.
Visual Illusions o Depend on experience. n Influenced by culture. o Illustrate normal perceptual processes. n These are not errors but rather failures of perception in unusual situations.
Visual Pattern Recognition o Bottom-up approaches: n n n Template-matching Feature analysis Recognition by components
Template-Matching o A retinal image of an object is compared directly to stored patterns (templates). n n The object is recognized as the template that gives the best match. Used by computers to recognize patterns. o Evidence shows human recognition is more flexible than template-matching: n Size, place, orientation, shape, blurred or broken (ambiguous or degraded items easily recognized by people.
Feature Analysis o Stimuli are combinations of elemental features. n n Features are recognized and combined. Features are like output of edge detectors. o Features are simpler, so problems of orientation, size, etc. , can be solved. o Relationships among features are specified to define the pattern.
Evidence for Feature Analysis o Confusions – people make more errors when letters presented at brief intervals contain similar features: n G misclassified: as C (21), as O (6), as B (1), as 9 (1) o When a retinal image is held constant, the parts of the object disappear: n n Whole features disappear. The remaining parts form new patterns.
Object Recognition o Biederman’s recognition-by-components: n n n 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. o Only edges are needed to recognize geons.
Tests of Biederman’s Theory o Object recognition should be mediated by recognition of object components. o Two types of degraded figures presented for brief intervals: n n Components (geons) missing Line segments missing o At fast intervals (65 -100 ms) subjects could not recognize components when segments were missing.
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