Cognitive Processes PSY 334

1. Cognitive ProcessesPSY 334 Chapter 2 – Perception

2. 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.

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

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

5. 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.

6. 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.

7. Five Principles • Proximity • Similarity • Good continuation • Closure • Common fate • Elements that move together group together. • These will be on the midterm.

8. Examples • Gestalt principles of organization • Reversible figures • Stuart Anstis demos: http://psy.ucsd.edu/~sanstis/SACamov.html http://psy.ucsd.edu/~sanstis/motion.html

9. 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.

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

11. Visual Pattern Recognition • Bottom-up approaches: • Template-matching • Feature analysis • Recognition by components

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. Speech Recognition • The physical speech signal is not broken up into parts that correspond to recognizable units of speech. • Undiminished sound energy at word boundaries – gaps are illusory. • Cessation of speech energy in the middle of words. • Word boundaries cannot be heard in an unfamiliar language.

18. Phoneme Perception • No one-to-one letter-to-sound correspondence. • Speech is continuous – phonemes are not discrete (separate) but run together. • Speakers vary in how they produce the same phoneme. • Coarticulation – phonemes overlap. • The sound produced depends on the sound immediately preceding it.

19. Feature Analysis of Speech • Features of phonemes appear to be: • Consonantal feature (consonant vs vowel). • Voicing – do vocal cords vibrate or not. • Place of articulation – where the vocal track is constricted (where is tongue placed). • The phoneme heard by listeners changes as you vary these features. • Sounds with similar features are confused.

20. Categorical Perception • For speech, perception does not change continuously but abruptly at a category boundary. • Categorical perception – failure to perceive gradations among stimuli within a category. • Pairs of [b]’s or [p]’s sound alike despite differing in voice-onset times.

21. Two Views of Categorical Perception • Weak view – stimuli are grouped into recognizable categories. • Strong view – we cannot discriminate among items within such a category. • Massaro – people can discriminate within category but have a bias to same items are the same despite differences. • Category boundaries can be shifted by fatiguing the feature detectors.