Cognitive Processes PSY 334 Chapter 2 – Perception July 1, 2003

Visual Perception Distal stimulus -- tree Proximal stimulus – image of tree on retina Percept – interpretation of proximal stimulus as a tree Size and color constancy

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.

Marr Depth cues (texture gradient, stereopsis) – where are edges in space? How are visual cues combined to form an image with depth? Primal sketch – extracts features. 2-1/2 D sketch – identifies where visual features are in relation to observer (depth). 3-D model – refers to the representation of the objects in a scene, combines context.

Pattern Recognition Classification and recognition occurs through processes of pattern recognition. Bottom-up processes – feature detection Top-down processes -- conceptually driven processing

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.

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.

Examples Gestalt principles of organization Reversible figures

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.

Visual Illusions Depend on experience. Influenced by culture. Illustrate normal perceptual processes. These are not errors but rather failures of perception in unusual situations.

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.