Cognitive Processes PSY 334 Chapter 2 Perception Marr

Cognitive Processes PSY 334 Chapter 2 – Perception

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 q Proximity q Similarity o Good continuation o Closure o Common fate n Elements that move together group together. o These will be on the midterm.

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

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.

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

Visual Pattern Recognition o Bottom-up approaches: n n n Template-matching Feature analysis Recognition by components

Template-Matching o A retinal image of an object is compared directly to stored patterns (templates). n n The object is recognized as the template that gives the best match. Used by computers to recognize patterns. o Evidence shows human recognition is more flexible than template-matching: n Size, place, orientation, shape, blurred or broken (ambiguous or degraded items easily recognized by people.

Feature Analysis o Stimuli are combinations of elemental features. n n Features are recognized and combined. Features are like output of edge detectors. o Features are simpler, so problems of orientation, size, etc. , can be solved. o Relationships among features are specified to define the pattern.

Evidence for Feature Analysis o Confusions – people make more errors when letters presented at brief intervals contain similar features: n G misclassified: as C (21), as O (6), as B (1), as 9 (1) o When a retinal image is held constant, the parts of the object disappear: n n Whole features disappear. The remaining parts form new patterns.

Object Recognition o Biederman’s recognition-by-components: n n n 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. o Only edges are needed to recognize geons.

Tests of Biederman’s Theory o Object recognition should be mediated by recognition of object components. o Two types of degraded figures presented for brief intervals: n n Components (geons) missing Line segments missing o At fast intervals (65 -100 ms) subjects could not recognize components when segments were missing.

Speech Recognition o The physical speech signal is not broken up into parts that correspond to recognizable units of speech. n n n 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.

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

Feature Analysis of Speech o Features of phonemes appear to be: n n n 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). o The phoneme heard by listeners changes as you vary these features. n Sounds with similar features are confused.

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

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