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Cognitive Psychology: Sensation and Perception

A similar process happens between the letter-recognition and word-recognition levels. Activating the representation of a particular letter, such as a K, will also activate representations of words containing this letter. This activation, in turn, will be passed back to the letter-recognition level, resulting in greater sensitivity to other letters occurring in those words. Activation of these candidate letters, in turn, feeds back to the feature-analyzer level. Because both inhibition and activation occur in this system, features not contained in candidate letters, and letters not contained in candidate words, are suppressed.

When we combine words into sentences and sentences into paragraphs, a similar process takes place. Information at higher levels (e.g., knowledge of what a paragraph is about) aids the processing of information at lower levels (e.g., the meaning of a sentence). An interactive process like this seems to occur in other perceptual domains as well. These computations take place very rapidly and unconsciously. Deceptively simple experiments, such as those that revealed the word-superiority effect, indicate that some kind of interactive process occurs. This perspective on perception has been built into computer simulations of perceptual processing that use parallel rather than serial (one step at a time) processing and that permit both top-down and bottom-up flow of information. The new simulations perform well in perceiving words and speech in a manner resembling readers and listeners.

"Where" and "What" Modules of the Visual System

Within the past decade, researchers have discovered that the large proportion of the brain's cortex devoted to vision is organized into separate areas, each with a different function. In many cases, these cortical areas preserve what is known as a "retinotopic mapping." That is, when adjacent areas of the retina are stimulated, adjacent areas in a corresponding region of the cortex are activated as well. Over 30 distinct brain areas have been identified, underlying such functions as recognizing shapes, colors, motion, and even facial expression.

Research on perceptual deficits resulting from brain injury has revealed a major distinction between "what" (object identification) and "where" (location) functions. For example, people with lesions in the posterior parietal region of the cortex can recognize an object as a cup but may find it virtually impossible to use information about its location and orientation to shape their hand properly as they reach for its handle.

Evidence indicates specialized functions for identifying faces. Prosopagnosia is a disorder due to brain injury in which the patient loses the ability to recognize human faces but can still recognize other sorts of objects and can make fine visual distinctions, such as distinguishing among very similar eyeglass frames. In fact, there are at least three separate brain systems for processing faces: one processes facial identity (whose face is it?), one processes face familiarity (have I seen it recently?), and one processes facial expression (is it expressing happiness, anger, sadness?).

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