The rhythm of attentional stimulus selection during visual competition

Recent research indicates that attentional stimulus selection could in fact be a rhythmic process, operating as a sequence of successive cycles. When two items must be monitored, an intriguing corollary of this \"blinking spotlight\" notion could be that the successive cycles are directed alternately to each target; as a result, each item would effectively be selected at half the intrinsic rate of attentional selection. Here, we tested this prediction in two experiments. In an endogenous attention task, subjects covertly monitored one or two peripheral images in order to detect a brief contrast change. In the sustained occipital EEG power spectrum, selecting two vs. one item resulted in a relative increase around 4Hz and a relative decrease around 10-11Hz. In a second experiment, we tested if comparable oscillations could be observed in the stimulus-evoked EEG visual representational content. Subjects saw a first peripheral image displayed alone for 600ms, before a second one also appeared for the same duration, but at a different peripheral location. Using pattern analysis on EEG evoked-responses, we were able to create item selective classifiers that constantly indicated which stimulus was on the screen. The time-course of single-trial classifier decision values presented a relative spectral peak around 11Hz when only one object was present, and around 4-5Hz when two objects were on the screen. These results are both compatible with an attentional stimulus selection process sampling the visual field at around 10-11Hz, and resulting in a half-frequency effective sampling around 4-5Hz when there are two items to monitor.