Cognitive Processes PSY 334 Chapter 2 Perception April

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