Neuropsychology of Object and Face Perception

In addition to using animal models, as described in the last subsection, it is also possible to examine the neurophysiological underpinnings of object perception with the help of humans who have sustained damage to visual areas of their brains. Agnosia is the general term for patients who have lost the ability to visually recognize objects. Such patients retain normal visual acuity and memory—they can see objects and remember object labels (as well as their structural and functional properties)—but they cannot retrieve an object's name from its image. Even more remarkably, patients with associative forms of agnosia may be able to produce excellent drawings of objects frommemory, but fail to name their own drawings when shown them later.

Associative agnosic patients show myriad patterns of spared and impaired object perception abilities. Patients with alexia are unable to recognize letters, but recognition of many other objects appears normal. Prosopagnosia is a selective inability to identify individual humans from their faces with, again, many other types of objects remaining recognizable. Other patients have category-specific agnosias, only showing difficulty naming fruits and vegetables, animals, or some other class of objects. Still other patients show even more idiosyncratic deficits, for example, one well-known patient is excellent (by some tests, better than normal) at recognizing people by their faces but is severely agnosic for all other object classes.

The dissociation evidenced in the agnosia literature between the perception of faces and other objects (a number of patients showing impaired face recognition abilities with relatively intact object recognition, along with at least one patient showing the opposite pattern of abilities and disabilities) is especially interesting to object perception theorists. All faces have the same basic parts in similar relations to each other, so that it would be difficult to craft a structural description that could distinguish different faces from each other. Thus, a number of researchers have drawn the inference that faces are perceived by a visual subsystem that uses view-based representations and is damaged in prosopagnosic patients, whereas other objects are perceived by a structural-description-based subsystem that is relatively spared in prosopagnosia.

Several neuroimaging laboratories using techniques such as functional magnetic resonance imaging (fMRI) have showed that greater cellular activity is generated in a particular portion of IT cortex (more specifically, the anterior fusiform gyrus) when humans view pictures of faces than when pictures of other objects are viewed. The tantalizing conclusion is that this is the area of the brain where view-based representations of faces are processed and/or encoded and that selective injury to this area results in prosopagnosia. This hypothesis will be evaluated in greater detail later after we have considered behavioral evidence concerning the different forms of processing inherent in the perception of faces and other types of objects.

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