Visuomotor adaptation is impaired in patients with unilateral neglect.

Patients with unilateral neglect tend to ignore sensory information from their contralesional hemispace. Many symptoms of neglect can be reduced by exposing patients to rightward-shifting prism goggles. It was noted that the effects on neglect symptoms last for at least two hours. This seems surprising in light of the fact that the after-effect of prism adaptation in healthy subjects lasts only for a few trials. To account for this discrepancy Michel et al. (2003) referred to anecdotal observations which suggested that neglect patients show little awareness of prism-induced spatial errors. They argued that this lack of awareness might interfere with more conscious attempts to compensate for the prism goggles (called strategic control) and thereby enhance the effects of more implicit corrective mechanisms (called spatial realignment) leading to more pronounced and longer-lasting after-effects. We examined this hypothesis in a group of neglect patients, patients with right-hemispheric lesions but no neglect and a group of healthy age-matched controls. Our findings confirm that strategic control mechanisms are impaired in neglect patients. However, their after-effects seem neither reduced nor pathologically increased, thereby suggesting that the two mechanisms of prism adaptation, namely strategic control and spatial realignment are quite independent of each other. Furthermore we found that these deficits are quite specific for neglect since other patients with right-hemisphere lesions but no neglect are not impaired in this task. We discuss the implications of our findings for our understanding of visual neglect, prism adaptation and the perception and action model.