Higher Order Apperceptive Deficits

Many patients have been identified who, although still classified as having apperceptive agnosia, appear to have some residual visual processing ability and can copy and match objects to some degree. These patients have object recognition difficulty under challenging conditions and do somewhat better in more optimal conditions. For example, they are impaired at recognizing objects under poor lighting conditions when shadows are cast, creating misleading contours. An additional manipulation that proves difficult for these patients is recognition of foreshortened or degraded objects. They are also poor at recognizing objects from unusual viewpoints or unconventional angles relative to more standard viewpoints. The classification of these patients is unclear because there is ongoing debate regarding the mechanisms that give rise to such deficits and whether, indeed, these deficits arise from a common source. Warrington and colleagues argued that the failure to identify objects from unusual but not conventional views is consistent with a deficit that occurs once sensory information has been processed and is attributable to a problem in categorizing two instances of the same stimulus as identical. Some patients appear to be impaired at deriving viewpoint-independent representations, despite the fact that they are able to construct a viewpoint-dependent representation. This distinction between viewpoint-dependent and -independent representation parallels the distinction made by Marr in his well-known theory of vision.

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