Interhemispheric differences in extrastriate areas during visuo-spatial selective attention.

Functional asymmetries between hemispheres have been reported in relation to spatial and temporal information processing. Here we used functional magnetic resonance imaging to investigate the influence of task on activity in extrastriate areas during selective spatial attention. During bilateral visual stimulation, subjects attended either the left or the right hemifield. Within the attended side, the task was either to discriminate the orientation of the stimuli or to judge their temporal characteristics. The bilateral stimulation caused symmetric activation of the left and right occipitotemporal junction. Within these regions we investigated the modulatory effects attention and the effect of task upon these. A region of interest approach was used to compare activity in the two hemispheres. The signal at occipitotemporal junction was analyzed in a 2 x 2 x 2 factorial design, with attended side, type of task, and hemisphere as factors. We found that, in both hemispheres, activity was higher when attention was directed to the contralateral hemifield compared with the ipsilateral hemifield. However, the size of these contralateral attentional modulations was dependent on the task. In the left occipitotemporal junction, contralateral modulations were stronger during the temporal task, while in the right occipitotemporal junction contralateral modulations were stronger during orientation discrimination. Overall, this pattern of activity lead to a significant three-way interaction between attended side, type of task, and hemisphere. We conclude that task characteristics influence brain activity associated with spatial selective attention. Our results support the hypothesis that temporal and orientation processing are preferentially associated with the left and right hemisphere, respectively.