When one of the two hemispheres directs visuospatial attention to some contralateral location, this may imply that inhibition is exerted on the other hemisphere. In the dual rapid serial visual presentation task, two targets (T1 and T2) occur among two different visual streams, rapidly presented left and right. We replaced the standard letter distractors in one stream by "easy symbols" (ESy; easily distinguished from targets), to facilitate T1 identification. If interhemispheric inhibition is deployed and released, this facilitation of one hemisphere's task should reduce its inhibitory effect on the other hemisphere, leading to improved T2 identification by the other hemisphere. This prediction was confirmed in three experiments. Furthermore, event-related EEG potentials did not only show a constant bias of allocated attention between ESy and standard streams, with letters evoking visual potentials earlier than ESy, but also short-term enlargement of this effect in the pair of distractors that followed T1. Taken together, these results support the notion of interhemispheric inhibition in visuospatial attention.
