Attention searches nonuniformly in space and in time

Significance Visual search is one of the primary tools to study attention in experimental psychology, yet the spatiotemporal behavior of attention during search remains controversial: Does attention focus on one item at a time or several, or even all, items? We used an innovative methodology that relies on simple yet elegant mathematical reasoning and only assumes (like most other attentional studies) that attentional strength is reflected in behavioral performance. We provide evidence against a strict parallel model in space and for an attentional periodicity in time: Attention during a difficult search task was nonuniformly allocated both in space, focusing on one stimulus or subgroup of stimuli at a time, and in time, with this deployment being periodically modulated at ∼7 Hz. Difficult search tasks are known to involve attentional resources, but the spatiotemporal behavior of attention remains unknown. Are multiple search targets processed in sequence or in parallel? We developed an innovative methodology to solve this notoriously difficult problem. Observers performed a difficult search task during which two probes were flashed at varying delays. Performance in reporting probes at each location was considered a measure of attentional deployment. By solving a second-degree equation, we determined the probability of probe report at the most and least attended probe locations on each trial. Because these values differed significantly, we conclude that attention was focused on one stimulus or subgroup of stimuli at a time, and not divided uniformly among all search stimuli. Furthermore, this deployment was modulated periodically over time at ∼7 Hz. These results provide evidence for a nonuniform spatiotemporal deployment of attention during difficult search.