Cognitive Processes PSY 334. Chapter 2 Perception

Transcription

1 Cognitive Processes PSY 334 Chapter 2 Perception

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

3 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.

4 Early Visual Processing Parts of the eye Two kinds of photoreceptors: Rods respond to motion, light & dark Cones respond to color, shape, detail Fovea is the area of the retina with highest resolution best for seeing detail. We move our eyes so light hits the fovea.

5 The Eye

6 Later Visual Processing Neural pathways from the eyes to the visual cortex split at the optic chiasm. Info from the left visual field goes to the right hemisphere. Info from the right visual field goes to the left hemisphere. Two pathways from the visual cortex: Where pathway What pathway

7 Pathways to the Visual Cortex

8 Pathways Forward Where is it? What is it?

9 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.

10 Edge and Bar Detectors

11 Computer Edge Detection

12 Feature Maps In addition to edges, lines, bars, other information is extracted from the visual signal: Color Motion These aspects, called features, are represented in feature maps located in different areas of the brain.

13 Depth Perception Our eyes turn a three-dimensional world into a two-dimensional image on the retina. Our cortex turns that two-dimensional image back into three-dimensions (depth). Cues are used to infer distance. Cues must be learned through experience. Depth cues in art:

14 Size Constancy is Mental

15 The same photo

16 The same photo again

17 Marr Depth cues (texture gradient, stereopsis, motion parallax) 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.

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

19 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.

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

21 Examples Gestalt principles of organization Illusory contours: Reversible figures Apparent motion demos:

22 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.

23 Visual Illusions Depend on experience. Influenced by culture. Illustrate normal perceptual processes. These are not errors but rather failures of perception in unusual situations. Try some yourself:

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

25 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.

26 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.

27 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.

28 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.

29 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 ( ms) subjects could not recognize components when segments were missing.

30 Face Recognition Prosopagnosia inability to recognize familiar faces. Are faces special? Thatcher effect Damage to fusiform gyrus causes prosopagnosia. The area may also be used for fine-grained distinctions needed to recognize faces but also other objects. Bird, car & greeble experts all use it.

31 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.

32 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.

33 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.

34 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.

35 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 say items are the same despite differences. Category boundaries can be shifted by fatiguing the feature detectors.

36 Top Down Processing General knowledge (context, high-level thinking) combines with interpretation of low-level perceptual units (features). Context limits the possibilities so fewer features must be processed: Word superiority effect D or K vs WORD or WORK words do 10% better. To xllxstxatx, I cxn rxplxce xvexy txirx lextex of x sextexce xitx an x, anx yox stxll xan xanxge xo rxad xt wixh sxme xifxicxltx.

37 Context and Speech Phoneme restoration effect: It was found that the *eel was on the axle. It was found that the *eel was on the shoe. It was found that the *eel was on the orange. It was found that the *eel was on the table. The identification of the missing word depends on what happens after it.

38 Faces and Scenes When parts are presented in isolation, more feature information is needed to recognize them. Face parts are recognized with less detail when in the context of a face. Subjects are better able to identify objects when they are part of coherent novel scenes rather than jumbled scenes.

39 Change Blindness People cannot keep track of all of the information in a complex scene. If change occurs during a scene-cut or eye movement and it fits the context, it may be missed. Large changes can be overlooked. 7 of 15 participants noticed that the person changed entirely while giving directions Demo:

40 Models of Object Perception Two competing models explain how context and feature information are combined: Massaro s FLMP (fuzzy logic model of perception) -- Context and detail are two independent sources of information. McClelland & Rumelhart s PDP model connectionist model in which both sources of information interact.

41 Testing the FLMP Model Four kinds of stimuli: Only an e can make a real word. Only a c can make a real word. Both letters can make a word. Neither letter can make a word. Within each group, stimuli go from e to c. Subjects saw each stimulus word briefly and had to identify the letter, e or c.

42 FLMP Results Observed frequencies for naming a letter e increase as it has more e features, but also as the context demands an e. Baye s theorem gives a formula for combining the independent contributions of two sources of information. Massaro s results conform to predictions of Baye s theorem, suggesting that the information sources must be independent of each other.

43 Testing the PDP Model Activation spreads from features to excite letters and from letters to excite words (bottom up processing). Activation also spreads from words to the component letters (top-down processing). The more activation, the more likely the correct letter will be identified: TRAP vs TRIP

44 Comparing the Two Models Subjects heard a phoneme that varied from r to an l in two contexts: A syllable beginning with t tr or tl. A syllable beginning with s sl or sr. Both the FLMP and PDP models were compared to actual subject data. FLMP was close to what subjects did. PDP was too strongly affected by context.

45 PDP Model Describes More The PDP model suggests that information is not separately processed but each letter affects each other letter. Recognition of a in MAVE is almost as good as recognizing it in MADE. This occurs because MAVE is similar to many other words with an A in that position. We do not have a context but four letters that each influence the others.

46 Marr Depth cues (texture gradient, stereopsis) where are edges in space? How are visual cues combined to form an image with depth? Primal sketch extracts features. 2-1/2 D sketch identifies where visual features are in relation to observer (depth). 3-D model refers to the representation of the objects in a scene, combines context.

47 Putting it All Together The output of these stages (see Fig 2.31) is a representation of an object and its location. This output is used as input to higherlevel cognitive processes. Conscious awareness (a higher-level process) involves the recognition stage, but lots of processing occurs first.