What Agnosia Tells Us About Normal Vision

A major obstacle to understanding object recognition is that we perform it so rapidly and efficiently that the outcome belies the underlying complexity. One approach to discovering the processes that mediate object recognition is to study the performance of individuals who have an impairment. This breakdown approach has proven extremely illuminating and has provided important insights into the mechanisms involved in normal object recognition. The breakdown approach as reflected in the study of neuropsycholo-gical patients with agnosia is related to other approaches that also examine the system in its nonoptimal state. These approaches include the study of visual illusions in which the perception of normal subjects is distorted through some stimulus manipulation and the study of perception when cues are reduced, such as in monocular versus binocular vision.

Neuropsychological studies of agnosia have not only identified a major distinction between "early" and "late" stages of object recognition, as well as differentiated more discrete impairments within each of these stages, but also uncovered deficits associated with "intermediate''-level vision. Additionally, investigations with patients have allowed us to address issues such as category specificity both within the domain of objects and across visual domains, relating faces and words to objects. Finally, how perception might be related to action has been a focus of neuropsychological research and important observations have been gleaned from the detailed and thorough study of these patients with agnosia.

Studies of patients with agnosia have also shed light on the extent to which there is modular organization in the visual system. Although we have been concerned only with deficits of object recognition following brain damage, there are also patients with selective deficits of depth, motion, and color processing. One interpretation of these selective deficits is that there are independent regions of the brain that are specialized for certain functions. An even more extreme view, but one that has been tempered recently, is that these independent regions are exclusively dedicated for particular visual functions. At a higher level, whether there are truly independent areas for recognition of different categories of visual objects (living/nonliving) or for different types of stimuli (faces, words, or objects) remains a matter of ongoing investigation.

Perhaps most important is that these studies of patients with visual object agnosia have constrained our theories of object recognition and, in turn, these theories have guided our investigation of these interesting and illuminating deficits.

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