Interhemispheric enhancement of somatosensory cortical excitability through contralateral repetitive transcranial magnetic stimulation.

OBJECTIVES Somatosensory evoked potentials (SEPs) were used to index somatosensory-somatosensory interhemispheric interactions and highlight potential mechanisms by which TMS alters contralateral somatosensory cortex excitability. METHODS Fifteen healthy individuals participated in three sessions on separate days. On each day participants received either: (1) continuous theta burst (cTBS), (2) 1 Hz repetitive transcranial magnetic stimulation (rTMS) or (3) control TMS over left somatosensory cortex. SEPs from right somatosensory cortex were recorded before and after TMS while participants were at rest, performed sensorimotor tracking or the sustained attention to response task (SART). Left-handed tracking performance was also indexed. RESULTS N20-P27 amplitude was increased following 1 Hz rTMS while participants were at rest. This increased amplitude was not observed during right-handed tracking or the SART. N20-P27 amplitude was not influenced by cTBS or control TMS. P15-N20 and N34-P50 SEP components were not influenced by TMS. Right- and left-handed tracking performance was not transiently influenced by TMS. CONCLUSIONS The results support TMS induced somatosensory-somatosensory interactions and offer converging evidence for an intercortical, rather that intracortical, mechanism that mediates contralateral sensory processing. These interactions appear to be dependent on concurrent attention/task demands. SIGNIFICANCE Somatosensory-somatosensory interactions are reflected by intercortical mechanisms that are state and task dependent.