Despite the behavioral differences between JW and CK, they have a dramatic deficit: They are unable to recognize even common, familiar objects—a disorder termed "agnosia" by Sigmund Freud (coined from the Greek ''without knowledge''). Visual agnosia is a disorder of recognition, in which a person cannot arrive at the meaning of some or all categories of previously known visual stimuli, despite normal or near-normal visual perception and intact alertness, intelligence, and language. Despite the visual recognition problems associated with agnosia, there is normal recognition of objects through modalities other than vision (touch, auditory, and verbal definitions or description of their function), which suggests that the deficit is not simply a difficulty in retrieving names or in accessing the necessary semantic information. Visual recognition has been more extensively studied than recognition in other modalities, although similar deficits have been observed in patients with auditory (auditory agnosia) or tactile (tactile agnosia) deficits.

The traditional view of agnosia as a specific disorder of recognition has undergone considerable challenge in the past, with critics contending that all visual agnosias can be explained by a subtle alteration in perceptual functions likely accompanied by a generalized intellectual deterioration. Despite this early skepticism, there is now widespread acceptance of this disorder as a legitimate entity and detailed case studies have been concerned with characterizing both the underlying mechanisms that give rise to this disorder and the overt behaviors.

Lissauer was the first to classify visual object agnosia into two broad categories: apperceptive ''mindblind-ness'' and associative mindblindness. These impairments were evaluated by requiring patients to (i) describe the formal features of a pattern, (ii) reproduce it by drawing, and (iii) recognize it among similar alternatives. Using Lissauer's classifications, a person with apperceptive agnosia is assumed to be impaired at constructing a perceptual representation from vision and subsequently is unable to copy, match, or identify a drawing. In contrast, a person with associative agnosia is one who cannot use the derived perceptual representation to access stored knowledge of the object's functions and associations but is able to copy and match the drawing even though he or she is unable to identify it.

Recent neuropsychological accounts by Humphreys and Riddoch as well as by Warrington and colleagues and computational accounts such as that of Marr and colleagues have sought to extend Lissauer's dichotomy for two reasons. The first reason is the growing understanding that visual object recognition comprises many distinct steps not captured by the simple dichotomy. For example, it has been suggested that apperceptive processes include encoding the primitive dimensions of shape and segmentation of figure from ground. Associative processes may also be subdivided to include access to stored visual knowledge of objects, followed by access to stored associative and functional (semantic) knowledge from the description derived from the image. The second reason for further differentiation of the underlying processes, and the lesion types, derives more fine-grained neuropsycho-logical analysis. One such example is of patients who show impaired access to knowledge of associative and functional properties of the object but have well-preserved understanding of the object's shape, as

Figure 2 Line drawings misnamed by patient CK.

reflected in a high-complexity object decision task (differentiating real objects from novel objects that are composed of parts of real objects). Other patients perform relatively poorly at object decision but are still able to carry out many high-level perceptual tasks, such as matching objects across different viewpoints and sorting pictures into basic categories. These developments have forced a further refinement of our understanding of visual processing and the types of breakdown that are possible. Despite the simplicity of Lissauer's dichotomy and its clear inadequacy, it provides a coarse framework that has proved useful in describing agnosia, as illustrated in the book Visual Agnosia by Farah. Following Farah, we adopt this dichotomy as a starting point and describe these two forms of agnosia, we also provide a detailed discussion of the patients described previously and the implications of such disorders for our further understanding of visual object recognition. Before we continue our exploration of these types of agnosia, however, we first identify the underlying neuromechanisms responsible for this visual perceptual processing.

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