Effects of paired pulse TMS of primary somatosensory cortex on perception of a peripheral electrical stimulus

Paired pulse transcranial magnetic stimulation (paired TMS) was introduced to study local inhibitory or facilitatory intracortical circuits of the primary motor cortex. However, similar interactions can be shown in other areas of cortex. The current study tests the effects of paired pulse TMS of the right primary somatosensory cortex (S1) on the sensory perception of electrical stimuli applied on the contralateral thumb finger. In the main experiment a subthreshold conditioning stimulus (CS) preceded a suprathreshold test stimulus (TS) at different inter-stimulus intervals. We found that perception of a peripheral electrical stimulus was markedly attenuated by paired TMS in comparison to single pulse TMS when the ISIs was 10 or 15 ms, while there was no effect at shorter ISIs. There was no additional effect of the CS pulse if the intensity of the TS was subthreshold. In control experiments we observed that the effect vanished when the delay between the peripheral stimulus and the TS was 10 or 30 ms rather than 20 ms or if the pairs of pulses were applied over the vertex rather than the hand area. Furthermore, there was no change at longer ISIs when paired TMS was applied over the posterior parietal cortex of the same hemisphere. These results demonstrate that paired pulse TMS is able to probe intracortical circuits in S1 and that the intrinsic properties of these circuits differ even between closely adjacent areas of the cortex.