IMPAIRMENT OF AN EGOCENTRIC MAP OF LOCATIONS - IMPLICATIONS FOR PERCEPTION AND ACTION

We report a patient (GW) with focal atrophy of the posterior, superior parietal lobes who exhibited a progressive impairment on tasks that d epend on spatial information. Clinical and electrophysiological assess ments of visual function demonstrated no clear abnormalities. Visual a ttention was relatively preserved in contrast to judgements of the loc ation of visually and auditorally presented targets, which were highly abnormal. Examination of her performance on pointing tasks demonstrat ed a remarkable dissociation: GW pointed relatively accurately to visu alised targets but was profoundly impaired when asked to point to the remembered locations of targets defined by either visual or propriocep tive input. We attribute this deficit to disruption of a spatial map t hat represents visual information accumulated across saccades in an eg ocentric co-ordinate system. These data suggest (1) that spatial atten tion and spatial representation are dissociable, and (2) that motor sy stems may be accessed by at least two distinct spatial representations , one in which retinotopic information is integrated with head and bod y position on-line and the other a longer-lasting egocentric represent ation.