BACKGROUND: Dual-site transcranial magnetic stimulation (ds-TMS) is a neurophysiological technique to measure functional connectivity between cortical areas.
OBJECTIVE/HYPOTHESIS: To date, no study has used ds-TMS to investigate short intra-hemispheric interactions between the somatosensory areas and primary motor cortex (M1).
METHODS: We examined somatosensory-M1 interactions in the left hemisphere in six experiments using ds-TMS. In Experiment 1 (n = 16), the effects of different conditioning stimulus (CS) intensities on somatosensory-M1 interactions were measured with 1 and 2.5 ms inter-stimulus intervals (ISIs). In Experiment 2 (n = 16), the time-course of somatosensoy-M1 interactions was studied using supra-threshold CS intensity at 6 different ISIs. In Experiment 3 (n = 16), the time-course of short-interval cortical inhibition (SICI) and effects of different CS intensities on SICI were measured similar to Experiments 1 and 2. Experiment 4 (n = 13) examined the effects of active contraction on SICI and somatosensory-M1 inhibition. Experiments 5 and 6 (n = 10) examined the interactions between SAI with either 1 ms SICI or somatosensory-M1 inhibition.
RESULTS: Experiments 1 and 2 revealed reduced MEP amplitudes when applying somatosensory CS 1 ms prior to M1 TS with 140 and 160% CS intensities. Experiment 3 demonstrated that SICI at 1 and 2.5 ms did not correlate with somatosensory-M1 inhibition. Experiment 4 found that SICI but not somatosensory-M1 inhibition was abolished with active contraction. The results of Experiments 5-6 showed SAI was disinhibited in presence of somatosensory-M1 while SAI was increased in presence of SICI.
CONCLUSION: Collectively, the results support the notion that the somatosensory areas inhibit the ipsilateral M1 at very short latencies.