Behavioral task. While maintaining fixation at the center of the screen, the subject was instructed to start attending to the left-central RSVP stream and to monitor for occasional targets (2, 3, #, $). When a shift cue was presented in the left-central stream, the subject had to shift attention covertly to the right-central RSVP stream, and vice versa, and continue to monitor for targets in the currently relevant stream. The subject had to press one of four buttons to indicate the identity of each target appearing in the relevant stream.