"Gating" effects of simultaneous peripheral electrical stimulations on human secondary somatosensory cortex: a whole-head MEG study.

The secondary somatosensory cortex (SII) is strongly involved in the processing of somatosensory tactile and nociceptive sensations. We investigated the effect on SII responses of simultaneous painful and nonpainful electrical stimulations delivered to the thumb and little finger. According to the "bimodal" (i.e., nociceptive, tactile) organization of SII, it was expected that simultaneous painful and nonpainful stimulations would lead to modality interference with a marked reduction ("gating") of somatosensory evoked fields (SEFs) generated in SII. Eight different stimulus conditions were studied. Two conditions were simultaneous "unimodal" (thumb and little finger nonpainful; thumb and little finger painful) and two conditions were simultaneous "bimodal" (thumb nonpainful and little finger painful; thumb painful and little finger nonpainful). As a reference, four conditions included stimulations at single sites (thumb nonpainful, little finger nonpainful, thumb painful, little finger painful). The gating phenomenon was defined as the percentage of difference between the intensities of SII activation after simultaneous compared to the sum of the separate stimulations. Results showed that simultaneous stimulations induced gating effects on SEFs generated by SII. No significant gating differences were observed after the two unimodal stimulations, suggesting a negligible effect of global energy on gating. Instead, the gating effects on bilateral SII activity were stronger after simultaneous bimodal when compared to unimodal stimulations. Our findings hint that there could be a greater level of integration/convergence of painful and nonpainful stimuli in SII with respect to SI. Future studies should explore if it could have an important role in exploring pain relief.