Cognitive Processes PSY 334 Chapter 2 – Perception April 7, 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.