Functional neuroanatomy of visual search with differential attentional demands: an fMRI study.

Visual search is characterized as efficient (RT independent of distractor number) or inefficient (RT increasing with distractor number). Our goal was to determine if any brain regions are differentially activated by the attentional demands of these two search modes. We used fMRI to examine activation patterns during search for a target among a radial array of several distractors that were manipulated to produce efficient or inefficient search. Distractors for inefficient search were either uniform or varied to manipulate difficulty due to perceptual priming. No brain regions were uniquely activated by efficient or inefficient search, although inefficient search generally produced greater activations. The main differences were found in clusters in the superior occipital and superior parietal regions, for which activations were substantially greater for inefficient search. Similar results were found for frontal regions, such as the inferior prefrontal, superior frontal, anterior insula, and supplementary eye field, as well as the right ventral lateral thalamus. For inefficient search, increasing task difficulty resulted in low accuracy, but no difference in RT or activations. A working memory task utilizing the same display and response mode, but not involving search, activated the same frontal-parietal network as inefficient search (more so for the more difficult inefficient condition). Thus, our results identify brain regions that are more heavily recruited under conditions of inefficient search, independent of task difficultly per se, probably due in part to attentional modulation involving demands of eye movements, working memory, and top-down controls, but do not reveal independent networks related to efficient and inefficient search.