Cognitive ProcessesPSY 334

Chapter 2 – Perception

June 30, 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? 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 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.