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 depend on spatial information. Clinical and electrophysiological assessments of visual function demonstrated no clear abnormalities. Visual attention was relatively preserved in contrast to judgements of the location of visually and auditorally presented targets, which were highly abnormal. Examination of her performance on pointing tasks demonstrated a remarkable dissociation: GW pointed relatively accurately to visualised targets but was profoundly impaired when asked to point to the remembered locations of targets defined by either visual or proprioceptive input. We attribute this deficit to disruption of a spatial map that represents visual information accumulated across saccades in an egocentric co-ordinate system. These data suggest (1) that spatial attention and spatial representation are dissociable, and (2) that motor systems may be accessed by at least two distinct spatial representations, one in which retinotopic information is integrated with head and body position on-line and the other a longer-lasting egocentric representation.