Spatial Awareness: A Function of the Posterior Parietal Lobe?

Chronic unilateral visuo-spatial neglect is usually ‘defined’ clinically as a condition in which patients ignore objects and events in contralesional space. As such, ‘it’ has been observed after lesion to many distinct right hemisphere loci (and much more rarely after left hemisphere lesion). After cortico-subcortical damage, left neglect can be consequent upon right posterior parietal (Vallar and Perani, 1986), right temporal (Samuelsson et al., 1997), and right frontal (e.g., Damasio et al., 1980; Husain and Kennard, 1996) lesions. Neglect has also been reported after subcortical lesions of the thalamus (e.g., Watson and Heilman, 1979; Rafal and Posner, 1987), the basal ganglia (e.g., Damasio et al., 1980; Ferro et al., 1987) and the white matter/internal capsule (e.g., Healton et al., 1982; Ferro and Kertesz, 1984). There is, nonetheless, reasonable consensus that severe visuo-spatial neglect is most typically associated with lesion of the supramarginal gyrus of the right inferior parietal lobule – Brodmann’s area 39/40 (Vallar, 2001). Consensus is, of course, no guarantee of truth and some students have even argued that the detailed manifestations of so-called visuospatial neglect are too variable (both withinand between-patients) to be meaningfully united into one clinical entity (Halligan and Marshall, 1992; Stone et al., 1998). A recent report by Karnath et al. (2001) confirms that unilateral visuo-spatial neglect (in patients who meet the customary clinical features thereof) can result from lesions of right temporal cortex and from lesions of right parietal cortex (Vallar, 2001). But, more controversially, Karnath et al. then make a partially novel functional and anatomical distinction between two types of spatial neglect. In “pure” spatial neglect there is purportedly loss of “spatial awareness” contralateral to the lesion; the locus of the responsible lesion is exclusively superior temporal, without extension into occipital cortex or the optic radiations (and hence the patients do not manifest visual field deficits). By contrast, in ‘impure’ spatial neglect, there is a disorder of “directly coding space for action” (but not apparently of spatial awareness); here the lesion includes the inferiorposterior parietal cortex but also extends into regions which, when damaged, produce visual field deficits (see also Karnath, 2001, for further discussion). The functional distinction that Karnath et al. (2001) draw has some similarities with the contrast that Poppelreuter (1917) made between the roles of attentional