Prefrontal dimension change-related activation differs for visual search in sparse and dense displays.

Changes of the target-defining feature dimension have previously been shown to elicit anterior prefrontal activation increases. In the majority of studies, this change-related activation was observed in the left lateral frontopolar cortex. In at least one study, however, right anterior prefrontal activation was observed. Unlike previous work which typically used dense visual displays, the latter study employed sparse displays. Display density is known to affect search efficiency, such that dense displays give rise to efficient and sparse displays give rise to inefficient search. We reasoned that different neural processes might be involved in eliciting attentional dimension changes in efficient and inefficient search, so that variation of display density would change the laterality of dimension change-related activation in the anterior prefrontal cortex. We found that changes in the target-defining feature dimension selectively elicited right frontopolar activation during search in sparse displays, but not during search in dense displays, whereas the reverse pattern was observed in left frontopolar cortex. Our results demonstrate that different neural processes are at work during search in sparse and dense displays, resolving an apparent discrepancy in reported dimension change-related activation.