A person with simultanagnosia can perceive the basic shape of an object but is unable to perceive more than one object, or part of an object, at a time. Thus, these patients appear to have limited ability to process visual information in parallel, although they are relatively good at identifying single objects. Farah distinguished between two forms of simultanagnosia according to whether the patients had lesions affecting the dorsal or ventral visual stream. Each is discussed in turn here.

Although a person with dorsal simultanagnosia is able to recognize most objects, he or she generally cannot process more than one at a time, even when the objects occupy the same region of space. These individuals often have counting deficits and their descriptions of complex scenes are slow and fragmentary. The underlying impairment in dorsal simulta-nagnosia appears to be a disorder of visual attention so severe that these individuals cannot explicitly report perceiving the unattended objects. Dorsal simultanag-nosia is often observed in the context of Baliant's syndrome, is accompanied by oculomotor deficits and optic ataxia, and results from a bilateral parietooccipital lesion.

Individuals with apperceptive agnosia and those with dorsal simultanagnosia share many characteristics. In some cases, they may act effectively blind, being unable to negotiate visual environments of any complexity, and their perception appears to be piecemeal and confined to a local part or region of the visual field. The piecemeal nature of their perception, however, differs in significant ways. In apperceptive agnosia, only very local contour is perceived, whereas in dorsal simultanagnosia whole shapes are perceived, but only one at a time. Individuals with apperceptive agnosia use color, size, and texture to guess at objects but cannot use shape information. In contrast, individuals with dorsal simultanagnosia have intact shape perception. In contrast to apperceptive agnosia, the deficit in dorsal simultanagnosia appears to be attention related rather than shape related.

A person with ventral simultanagnosia usually has a lesion to the left inferior temporooccipital region. Although such a patient is generally able to recognize a single object, he or she does poorly with multiple objects and with single complex objects, irrespective of their size. Although they cannot recognize multiple objects, they differ from individuals with dorsal simultanagnosia in that they can perceive multiple objects. These individuals can count scattered dots and, if given sufficient time, can also recognize multiple objects. They respond slowly and often describe explicitly individual elements of the picture without appreciating the whole scene. This is also true in reading, and these patients are classified as letter-by-letter readers because they only recognize one letter of a word at a time, and hence show a linear relationship between reading speed and the number of letters in a word.

A recent reconceptualization of simultanagnosia by Humphreys and colleagues suggests that the two different forms are well characterized in terms of impairments in constructing different forms of spatial representations. Although those with ventral lesions are limited in the number of parts of an object they can represent, those with dorsal lesions are limited in the number of separate objects they can represent.

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