Rightward shift in spatial awareness with declining alertness.

Although transient neglect of contralesional space occurs following damage to either hemisphere, persistent forms are overwhelmingly associated with right hemisphere lesions. This has led to the suggestion that impairments in other right hemisphere systems--in particular those that mediate alertness--may undermine recovery. Reductions in neglect severity with stimulation, exacerbation with sedatives and the poor performance of chronic neglect patients on sustained attention tasks are consistent with this view. However, the question of whether changes in alertness exert a specific influence over spatial attention--or simply improve performance across many domains--is difficult to address using only patient studies. Here, we examine this question with individuals from the healthy adult population. On certain spatial tasks, adults show a modest but reliable leftward attentional bias. On the basis of the neglect studies, we hypothesised that this bias would diminish--or even reverse--as alertness levels declined. In the first study, participants were asked to judge the relative lengths of the left and right sections of a line when sleep deprived and when well rested. A significant rightward shift in attention was associated with sleep deprivation. A rightward shift was also observed over the course of the session. The second study replicated this time-on-task effect. The results suggest that a diminution in alertness may be sufficient to induce a rightward shift in visual attention in the healthy brain. Implications for the persistence of neglect in patients, for spatial biases in children and for normal free viewing asymmetries are discussed.