Brain activations during visual search: contributions of search efficiency versus feature binding

We investigated the involvement of the parietal cortex in binding features during visual search using functional magnetic resonance imaging. We tested 10 subjects in four visual search tasks across which we independently manipulated (1) the requirement to integrate different types of features in a stimulus (feature or conjunction search) and (2) the degree of search efficiency (efficient or inefficient). We identified brain areas that were common to all conditions of visual search and areas that were sensitive to the factors of efficiency and feature binding. Visual search engaged an extensive network of parietal, frontal, and occipital areas. The factor of efficiency exerted a strong influence on parietal activations along the intraparietal sulcus and in the superior parietal lobule. These regions showed a main effect of efficiency and showed a simple effect when inefficient conditions were compared directly with efficient pop-out conditions in the absence of feature binding. Furthermore, a correlation analysis supported a tight correspondence between posterior parietal activation and the slope of reaction-time search functions. Conversely, feature binding during efficient pop-out search was not sufficient to modulate the parietal cortex. The results confirm the important role of the parietal cortex in visual search, but suggest that feature binding is not a requirement to engage its contribution.