Fixating targets in visual search: The role of dorsal and ventral attention networks in the processing of relevance and rarity.

The dorsal attention network, often observed to be activated in serial visual search tasks, has been associated with goal-directed attention, responsible for the processing of task relevance. In serial visual search, the moment of target detection constitutes not only a task-relevant event, but also a rare event. In the present fMRI experiment, we disentangled task relevance from item rarity using a fixation-based analysis approach. We used a multiple target search task, and participants had to report the number of targets among distractors in the display. We had also added rare distractors to the displays. We found that rare events (targets and rare distractors) activated the dorsal attention network more strongly than common distractors. More importantly, we observed that the left IPS and the left insula, belonging to the dorsal and ventral attention system, respectively, were more strongly activated for targets compared to rare distractors. Using multi-voxel pattern analysis, we found that activation in the TPJ, bilaterally, an area also associated with the ventral attention system, distinguished between target and rare distractor fixations. These results point to an expanded role of the TPJ that seems to process post-perceptual information which is linked to task relevance.