SPATIAL ORIENTATION AND THE REPRESENTATION OF SPACE WITH PARIETAL LOBE LESIONS

Damage to the human parietal cortex leads to disturbances of spatial p erception and of motor behaviour. Within the parietal lobe, lesions of the superior and of the inferior lobule induce quite different, chara cteristic deficits. Patients with inferior (predominantly right) parie tal lobe lesions fail to explore the contralesional part of space by e ye or limb movements (spatial neglect). In contrast, superior parietal lobe lesions lead to specific impairments of goal-directed movements (optic ataxia). The observations reported in this paper support the vi ew of dissociated functions represented in the inferior and the superi or lobule of the human parietal cortex. They suggest that a spatial re ference frame for exploratory behaviour is disturbed in patients with neglect. Data from these patients' visual search argue that their fail ure to explore the contralesional side is due to a disturbed input tra nsformation leading to a deviation of egocentric space representation to the ipsilesional side. Data further show that this deviation follow s a rotation around the earth-vertical body axis to the ipsilesional s ide rather than a translation towards that side. The results are in cl ear contrast to explanations that assume a lateral gradient ranging fr om a minimum of exploration in the extreme contralesional to a maximum in the extreme ipsilesional hemispace. Moreover, the failure to orien t towards and to explore the contralesional part of space appears to b e distinct from those deficits observed once an object of interest has been located and releases reaching. Although patients with neglect ex hibit a severe bias of exploratory movements, their hand trajectories to targets in peripersonal space may follow a straight path. This resu lt suggests that (i) exploratory and (ii) goal-directed behaviour in s pace do not share the same neural control mechanisms. Neural represent ation of space in the inferior parietal lobule seems to serve as a mat rix for spatial exploration and for orienting in space but not for vis uomotor processes involved in reaching for objects. Disturbances of su ch processes rather appear to be prominent in patients with more super ior parietal lobe lesions and optic ataxia.