An attentional grasp reflex in patients with Alzheimer's disease.

Motor and visual grasp reflexes often occur as part of the symptomatology of Alzheimer's disease (AD). Similar grasp reflexes may also be associated with the impaired performance of AD patients on tasks which require direction of visual attention without eye movements. The covert orienting of visual attention task (COVAT) requires subjects to keep their eyes fixed on a central point and manually respond to the appearance of a peripheral target in one of two locations in the left or right visual field. Before the target appears, a cue is presented at either the target location or the contralateral location to the target, although the nature of the cue is not known until the target appears. We hypothesised that the attentional grasp reflex would become evident in patients with AD when attentional shifts away from the cue were required but there was no target for the shift present. Twelve patients who met the clinical criteria for AD and 12 age and education matched controls were administered three COVAT conditions in which the target appeared at the cued location with either an 80, 50 or 20% probability. For the 80 and 50% probability conditions. RTs for targets at the cued location were significantly faster than RTs for targets contralateral to the cue in both AD subjects and controls indicating that AD subjects were able to disengage attention when there was a target for the attentional shift. For the 20% probability condition, control subjects showed significantly faster RTs for targets appearing contralateral to the cue than for targets at the cued location indicating that when they expected the target to appear in the opposite visual field to the cue, they could initiate the shift of attention before the target appeared. However, AD subjects continued to show significantly faster RTs for targets at the cued location than for targets appearing contralateral to the cue, despite the high probability that targets would not occur at the cued location. This suggests that the covert redirection of attention away from a peripheral visual cue could not be initiated until the contralateral target appeared. Taken together these results suggest an attentional grap reflex in patients with AD.