Leftward bias in orienting to and disengaging attention from salient task-irrelevant events in rapid serial visual...

When embedded in the left or right stream of rapidly changing distractors, the second target (T2) is systematically better identified on the left than on the right. This left visual field advantage (LVFA) was recently attributed to better abilities of the right hemisphere in stimulus-driven orienting of attention: it was almost absent when salient uninformative cues were valid (presented in the T2 stream), and increased with invalid (different-stream) cues. However, cue-evoked negativity of event related potentials being earlier at the right than at the left hemisphere suggested that cues also are unequally processed, thereby possibly contributing to increased LVFA after invalid cues. This might occur through easier directing of attention toward left than right cues and/or through harder disengaging of attention from left than right cues. Alternatively, the increase in the LVFA could be caused by larger spatial distance between cues and T2 with invalid cues than with neutral cues presented at fixation. In order to test these hypotheses, an additional stream of stimuli at the vertical midline was used to separate the processing of lateral cues from the processing of lateral T2. If left cues increase the LVFA then this bias should be larger after invalid lateral than invalid midline cues and also midline T2 should be more impaired after left than right cues. These expectations were confirmed. Furthermore, increased negative amplitudes evoked by right cues suggest that orienting was more difficult toward right than left cues, and increased amplitudes of a following positivity suggest that disengaging attention was more difficult from left than right cues. Overall, these results suggest asymmetric abilities of the hemispheres in attentional processing of both task-relevant and salient task-irrelevant events.