Target side and scalp topography of the somatosensory P300

We recorded somatosensory P300 to target electrical shocks (10% probability) delivered to the left or right hand concurrently with non-target stimuli applied to the opposite hand. Target stimuli evoked a widely distributed P300, which was found to be double-peaked in 60% of the subjects investigated. The mean latencies of the "early" and "late" P300 peaks were respectively 302 +/- 8 msec and 353 +/- 12 msec. Both components (but especially the earlier one) were consistently lateralized over the hemiscalp contralateral to the target stimuli, whichever the target side. Although on theoretical grounds P300 lateralization might depend upon the sustained attentional activation of the hemisphere contralateral to targets, this hypothesis was contradicted in one complementary experiment in which we dissociated the side receiving the P3-evoking, rare, stimuli from the side to which sustained attention was directed. Under these conditions the P3 evoked by rare, but ignored, shocks corresponded to the "early" peak of target P300 and was also predominant contralateral to the evoking stimulus, i.e., over the hemisphere not activated by sustained attention. Therefore the lateralization of the somatosensory P300, or at least that of its "early" component, is that of the hemisphere receiving the P3-evoking volley, and not that of the one involved in maintaining sustained attention. The P3 to rare-ignored shocks and the "early" P300 to targets had identical latencies; both are likely to reflect an automatic processing of deviant stimuli and thus belong to the family of "novelty P3s" (or "P3a").