Interhemispheric differences in extrastriate areas during visuo-spatial selective attention

Functional asymmetries between hemispheres have been reported in relation t o spatial and temporal information processing. Here we used functional magn etic resonance imaging to investigate the influence of task on activity in extrastriate areas during selective spatial attention. During bilateral vis ual stimulation, subjects attended either the left or the right hemifield. Within the attended side, the task was either to discriminate the orientati on of the stimuli or to judge their temporal characteristics. The bilateral stimulation caused symmetric activation of the left and right occipitotemp oral 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 occipito temporal junction was analyzed in a 2 x 2 x 2 factorial design, with attend ed side, type of task, and hemisphere as factors. We found that, in both he mispheres, activity was higher when attention was directed to the contralat eral hemifield compared with the ipsilateral hemifield. However, the size o f these contralateral attentional modulations was dependent on the task. In the left occipitotemporal junction, contralateral modulations were stronge r during the temporal task, while in the right occipitotemporal junction co ntralateral modulations were stronger during orientation discrimination. Ov erall, 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 orientatio n processing are preferentially associated with the left and right hemisphe re, respectively. (C) 2000 Academic Press.