Unlike apperceptive agnosia, a person with associative agnosia can make recognizable copies of a stimulus that he or she may not recognize subsequently and can also successfully perform matching tasks. Teuber elegantly referred to this deficit as "perception stripped of meaning.'' As in the case of apperceptive agnosia, recognition is influenced by the quality of the stimulus and performance on three-dimensional objects is better than on photographs, and performance on photographs is better than on line drawings. The recognition deficit appears to result from defective activation of information pertinent to a given stimulus. There is a failure of the structured perception to activate the network of stored knowledge about the functional, contextual, and categorical properties of objects that permit their identification. In effect, this is a deficit in memory that affects not only past knowledge about the object but also the acquisition of new knowledge. Unlike individuals with apperceptive agnosia, who guess at object identity based on color and texture cues, people with associative agnosia can make use of shape information. When these individuals make mistakes in object identification, it is often by naming an object that is similar in shape to the stimulus. For example, FZ, a patient of Levine, misidentified a drawing of a baseball bat several times. Interestingly, his answer differed on each occasion, referring to it as a paddle, a knife, or a thermometer.
Although these patients can copy drawings well, the drawings are not necessarily normal; the end product might be a fairly good rendition of the target but the drawing process is slow and slavish and they can lose their place if they take their pen off the paper since they do not grasp component shapes. As can be seen in Fig. 4, a copy of a geometric configuration by patient CK, an individual with associative agnosia, was reasonably good, although the process by which he copied indicates a failure to bind the contours into meaningful wholes.
One of the important claims of associative agnosia is that perception is intact and it is meaning that is inaccessible. Much effort has been directed at evaluating this claim and the general finding is that even patients with associative agnosia have some form of visual impairment. For example, LH, a well-known and thoroughly documented agnosic patient studied by Levine and Calvanio, was moderately impaired on several tests of perception. He was considerably slower than normal subjects in making rapid comparisons between shapes or in searching for a prespecified target figure. His performance was also poor on tasks that required him to identify letters that were fragmented or degraded by visual noise, relative to control subjects. Based on the findings from LH and other associative
Original CKfs Copy
agnosic patients, it is clear that their perception is not normal. Although it may be considerably better than that of apperceptive agnosic patients, it is still impaired to some extent.
The brain damage in associative agnosia is more localized than in apperceptive agnosia. Some cases appear to involve only unilateral damage to the occipital lobe and bordering posterior temporal or parietal lobe. The lesions are often more circumscribed, sometimes involving the left inferior longitudinal fasciculus, which connects fusiform gyrus to temporal structures, or the bilateral posterior hemispheric areas in posterior cranial artery territory.
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