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Tracing attentional bias in visual perception

The right hemisphere plays a dominant role in some aspects of attentional processing. This can be inferred from performance of patients with unilateral spatial neglect, as well as from performance on attentional tasks of healthy subjects studied with imaging technics. However, since there is very little evidence of leftward bias in attention as measured in behavioral tasks, the question arises whether the lateralization of attention has any consequences for everyday life situations. In order to answer this question we developed a version of the two stream rapid serial visual presentation task. In this task, two targets are embedded in left and right simultaneous streams of rapidly changing letters. As has been shown in previous studies, the second target (T2) is identified better in the left than in the right visual field. In few experiments we studied the cognitive mechanisms of this leftward bias in T2 identification as well as its generality. We demonstrated that this leftward bias is related to stimulus-driven orienting of spatial attention rather than to temporal attention or lateralization of processing of alphanumerical stimuli in the left hemisphere. Furthermore, this leftward bias has been significantly reduced by endogenous orienting of attention. Notably, the leftward bias was not influenced by reading direction habits or handedness. We thus propose that this leftward bias is related to faster selection of relevant events by the right hemisphere rather than by the left hemisphere, as reflected by shorter latencies of T2-evoked N2pc component and of visual potentials evoked by the flow of background stimuli.