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Cognitive Processes PSY 334 Chapter 2 – Perception April 9, 2003.

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1 Cognitive Processes PSY 334 Chapter 2 – Perception April 9, 2003

2 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.

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

4 Visual Pattern Recognition  Bottom-up approaches: Template-matching Feature analysis Recognition by components

5 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.

6 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.

7 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.

8 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.

9 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.

10 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.

11 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.

12 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.

13 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.

14 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.

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