When is a view unusual? A single case study of orientation-dependent visual agnosia.

Several hypotheses have been proposed to explain why some neurological patients fail to recognise objects from unusual views: that it results from difficulty identifying an object's principal axis when it is foreshortened, identifying an object when landmark features are occluded, or an inability to rotate mental images. It was possible to test these hypotheses by examining the recognition abilities of a single case (A.S.), by using stimuli that were "unusual" only because of picture-plane misorientation. A.S. showed a recognition deficit in which his accuracy was proportional to the extent of misorientation from the normal upright for the object-although both principal axis and feature information remain visible after picture-plane rotation. Furthermore, A.S. performed with normal accuracy, and normal pattern of reaction time performance, on tasks of mental rotation. These findings suggest that none of these traditional accounts can adequately explain why this patient was unable to recognise objects from unusual views. These findings are discussed the light of recent suggestions of the basis of this type of disorder.