ABSTRACT
Despite having a history as long as that involving any cognitive disorder, neurological impairments of visual perception-the impairments underlying the syndrome of visual agnosia-remain poorly understood. This is likely due to several factors. One is that frank disorders of visual perception, unaccounted for by, for example, loss of acuity or generalized dementia, remain clinically rare. For this reason alone, some writers have even doubted their existence. For instance, writing in only 1972, Bender and Feldman claimed that visual agnosia did not exist, independent of problems in basic perceptual functions or in general cognitive function. Today, the case for the existence of visual agnosia as a clinical phenomenon is more secure. There exist several good single-case and group studies of patients with selective problems in particular perceptual processes mediating visual object recognition, which cannot be attributed to impaired basic perceptual or general cognitive functions (e.g., Grailet, Seron, Bruyer, Coyette, & Frederix, 1990; Humphreys & Riddoch, 1987b; Ratcliff & Newcombe, 1982; Riddoch & Humphreys, 1987a, 1987b; Warrington & Shallice, 1984; Warrington & Taylor, 1978). Perhaps even more serious as far as understanding visual agnosia is concerned is that, even when patients can be shown to have normal acuity and to be intellectually intact, attempts to link disorders to explicit models of visual perception have made only slow progress. Our thesis is that there needs to be close interlinking of neuropsychological studies with detailed computation-
al models of normal cognitive function, in order to understand a given neuropsychological disorder and in order to use neuropsychological evidence to evaluate particular models. We illustrate this by a discussion of both prior and current theoretical accounts of agnosia. We then attempt to go some way beyond this with reference to three single cases we have worked on, each of which (we argue) has a selective disturbance of a particular perceptual process. To account for such patients, the nature of these different processes must be articulated.
