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

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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

11. 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.

12. 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.

13. 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.

14. 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 higher-level cognitive processes.
Conscious awareness (a higher-level process) involves the recognition stage, but lots of processing occurs first.