Cognitive Processes PSY 334 Chapter 2 Perception April

Cognitive Processes PSY 334 Chapter 2 – Perception April 11, 2003

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.

Top Down Processing o General knowledge (context, high-level thinking) combines with interpretation of low-level perceptual units (features). o Context limits the possibilities so fewer features must be processed: n n 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 xanxge xo rxad xt wixh sxme xifxicxltx.

Context and Speech o Phoneme restoration effect: n n 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. o The identification of the missing word depends on what happens after it.

Faces and Scenes o When parts are presented in isolation, more feature information is needed to recognize them. n n 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.

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

Testing the FLMP Model o Four kinds of stimuli: n n 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. o Within each group, stimuli go from e to c. o Subjects saw each stimulus word briefly and had to identify the letter, e or c.

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

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

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

PDP Model Describes More o The PDP model suggests that information is not separately processed but each letter affects each other letter. n n 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. o We do not have a context but four letters that each influence the others.

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 n 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.

Putting it All Together o The output of these stages (see Fig 2. 31) is a representation of an object and its location. o This output is used as input to higherlevel cognitive processes. o Conscious awareness (a higher-level process) involves the recognition stage, but lots of processing occurs first.