Effects of tactile interference stimulation on somatosensory evoked magnetic fields following tibial nerve stimulation

We studied the effects of interfering tactile stimulation applied to the foot ipsilateral and contralateral to the stimulation on somatosensory evoked magnetic fields (SEFs) following tibial nerve stimulation at the ankle. Equivalent current dipoles (ECDs) of all 4 components, 1M–4M, in all sessions were estimated to be very close each other, around the foot area of the primary sensory cortex (SI). The IM, 2M and 4M components were significantly reduced in amplitude by the ipsilateral-foot interference, and we consider that this phenomenon is due mainly to `saturation' of the neurons in area 3b of the SI. In contrast, the 3M component was significantly enhanced in amplitude by the contralateral-foot interference. We suspect that this result was due to the effects of neuronal activities in areas 2, 5 and/or 7, which receive inputs from both sides of the body, i.e. to `bilateral function'. Considering the various types of interference effects on SEFs and somatosensory evoked potentials (SEPs) observed in not only the present, but also in the previous studies, we conclude that both SEFs and SEPs following tibial nerve stimulation are generated mainly by ascending signals mediated by cutaneous fibers of the peripheral nerves rather than the muscle afferents.