The influence of distracters, stimulus duration and hemianopia on first saccade in patients with unilateral neglect.

The aim of the present study was to assess the influence of visual distracters, stimulus duration and the presence of contralesional hemianopia on direction of the first saccade in right brain damaged (RBD) patients affected by left unilateral neglect (UN). During a visual search task we recorded eye movements in five RBD patients with UN and hemianopia (N+H+), nine RBD patients with UN but no hemianopia (N+H-), four RBD patients with neither neglect nor hemianopia and four normal controls. Two task variables were orthogonally manipulated: (a) presence or absence of distracters and (b) short or long stimulus duration. A significant interaction was found between groups, presence of distracters, stimulus duration and the direction of the first saccade made in the search. Independently of the temporal duration of targets, in N+H+ patients the presence of distracters enhanced the frequency of saccades directed ipsilesionally (i.e., rightward). In N+H- patients, distracters biased the first saccade toward the right side only at short stimulus duration. These data show that bias of attentional orienting toward stimuli in the right half of space is specific of UN. This pathological mechanism (a) is enhanced and prolonged, over the period of exploration, by concomitant complete contralateral hemianopia and (b) is most evident, even in the absence of concomitant visual field defects, when voluntary planning of attention and eye movements are precluded by the short duration of stimuli to be inspected.