Action control in visual neglect.

Abstract

Patients with unilateral neglect show a variety of impairments when reaching towards objects in contralesional space. The basis of these deficits could be perceptual, motor or at one of the intermediate stages linking these processes. Here, we review studies of visually guided reaching in neglect and integrate these results with findings from normal human and monkey action control. We consider evidence which shows that neglect patients can be slow to initiate or execute reaches particularly to a contralesional target. We discuss the directional and spatial deficits that may interact to contribute to such reaching abnormalities and highlight the importance of effective target selection and on-line guidance, exploring the idea that deficits in these mechanisms underlie increased susceptibility to ipsilesional visual distraction in neglect. We also examine the relationship between optic ataxia and neglect by considering two illustrative cases, one with pure optic ataxia and the other with optic ataxia plus neglect, which reveal differences in the anatomical substrates of the two syndromes. We conclude that many patients with neglect make abnormal visually guided reaches, but the pattern of reaching deficits is highly variable, most likely reflecting heterogeneity of lesion location across subjects. Rather than being specific to the neglect syndrome, abnormalities of reaching in these patients may correspond to the extent of damage to the visuomotor control system which involves critical regions in both the parietal and frontal cortex, the white matter tracts connecting them and subcortical regions. Thus, the action control deficits in neglect may be conceptualised as a range of impairments affecting multiple stages in the visuomotor control process.