Motor aspects of hemispatial neglect.

Abstract

A relatively common outcome of unilateral brain damage in humans is the syndrome of hemispatial neglect. Hemispatial neglect can occur following damage to a variety of brain regions but is chiefly associated with damage to the inferior parietal lobule, most frequently involving the occipito-temporo-parietal junction of the right hemisphere [1]. Neglect patients fail to respond appropriately to stimuli or events occurring within their contralesional hemispace, and may restrict eye and hand movements to objects or events located within ipsilesional space. Such behaviours are generally thought to result from an impairment in the ability to construct an appropriate representation of corporeal and extrapersonal space, or, as a consequence of an attentional bias which favours the processing of ipsilesional stimuli. However, neglect patients may also suffer from a disorder of ‘intention’, experiencing difficulty initiating movements towards targets presented within their neglected hemispace. It is the nature of any motor impairments associated with hemispatial neglect that is the focus of this special issue of Behavioural Neurology. While it is generally accepted that the syndrome of hemispatial neglect may reflect several underlying impairments, there has been a longstanding and influential view that a substantial number of neglect patients present with some form of motor bias. Right hemisphere patients presenting with left neglect have been shown to be slow in initiating leftward compared to rightward movements [e.g., 2,3], however, such studies have proven difficult to interpret as the movements in these studies are invariably cued by visual targets. Thus, it remains highly plausible that much of the motor impairment observed in hemispatial neglect has a perceptual basis, resulting in an impairment in the representation of space used to guide action [4]. If it is indeed the case that some aspects of hemispatial neglect may be attributable to a distorted representation of corporeal and extrapersonal space, then a critical question is whether such distorted perceptions lead to erroneous visuomotor behaviour. According to Milner and Goodale’s influential ‘two visual systems’ account [5], such perceptual distortions should affect perceptual judgments (e.g., explicit judgments of object size), but would be expected to have minimal effect on visuomotor actions directed to these same objects. This issue is explored further in the papers by McIntosh et al., Harvey et al., and Gore et al. which each examine the effects of egocentric location on the kinematics of reach-to-grasp movements and report the relative sparing of visuomotor performance. A related aspect of motor bias in hemispatial neglect concerns the paucity of leftward movements exhibited by neglect patients during spatial search tasks [6,7]. Thus, during both visual [6] and tactile [7] search tasks, the centres of exploration of space are shifted towards the right in neglect patients for both exploratory eye and hand movements. This issue is re-examined in the papers by Behrmann et al., and di Pelligrino et al. Behrmann et al., investigate whether the paucity shown by neglect patients in executing leftward saccades during spatial search tasks reflects a deficit in perception, in saccade planning, or in saccade execution. They report that patients with left neglect are impaired in planning but not in executing the contralesional saccades, and suggest that parietal cortex may play a specific role in directing action to different sides of space. The topic of ocular exploration of space is