1. Cognitive Processes PSY 334
Chapter 2 – Perception
April 7, 2003

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

3. 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.

4. 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.

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

6. Object Recognition Two stages:
Early phase – shapes and objects are extracted from background.
Later phase – shapes and objects are categorized, recognized, named.

7. 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.

8. 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.

9. Five Principles Proximity Similarity Good continuation Closure
Common fate
Elements that move together group together.

10. Examples
Gestalt principles of organization
Reversible figures

11. 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.