Abstract

Searching for an object in a complex visual scene involves selection mechanisms. Generally, it is assumed that efficient "pop-out" search involves mainly bottom-up processing, whereas inefficient search requires pronounced top-down control over visual processing. We used functional magnetic resonance imaging in behaving monkeys to explore the functional network involved in efficient visual search. As a pop-out target automatically attracts spatial attention, we attempted to determine the regions involved in feature selection independently of the spatial allocation of attention. Therefore, monkeys were trained to perform a search task in which they had to covertly detect the presence of a salient target among distractor objects. Three tasks were used to control, as much as possible, for the spatial allocation of attention. These control tasks were matched with the search task for visual input and manual responses. Pop-out search, when compared with the control tasks, activated 3 frontal regions: frontal eye field, area 45, and a posterior portion of area 46, in addition to small activation sites in lateral intraparietal area and inferotemporal area TE. Our results show that efficient search involves frontal regions as much as visual regions and in particular that ventral prefrontal area 45 is involved in top-down control during efficient search.