Cortical response to task-relevant stimuli outside the focus of attention.

Selective visual attention enhances the neural response to task-relevant visual items.  Responses to task-irrelevant and therefore presumably unattended items are not enhanced, or even suppressed relative to baseline. However, it is unknown what happens to items outside the focus of attention that are nevertheless relevant for the task at hand. We studied the retinotopic neural correlates of such processing with a dual-task fMRI-experiment. An attention-demanding central task was combined with one of two peripheral tasks concerning the same visual stimulus; one posing low and the other high attentional demands. Task-relevance increased BOLD-responses to the peripheral stimulus in the context of both task.  For the low-demand task, this increase was accompanied by good behavioral performance. For the high-demand task, performance remained near chance.  When the focus of attention was allowed to shift to the peripheral stimulus, neural responses increased even further. In a control experiment, we observed the differential persistence in iconic memory of visual attributes relevant to high- and low-demand tasks, respectively.  We conclude that, in the dual-task situation, the focus of attention initially remains on the central task, but subsequently shifts to the former location of the peripheral target.  This belated shift to an iconic memory explains the enhanced BOLD-response observed with both tasks.  Differential iconic persistence accounts for the disparate behavioral performance.   Our results suggest that attentional modulation, but not behavioral performance, is consistently associated with an enhanced BOLD-response.