MAGNETOENCEPHALOGRAPHIC STUDY OF INTRACEREBRAL INTERACTIONS CAUSED BYBILATERAL POSTERIOR TIBIAL NERVE-STIMULATION IN MAN

We studied somatosensory evoked magnetic fields (SEFs) following stimu lation of bilateral posterior tibial nerves ('bilateral' waveform) in normal subjects to determine the inter- and intra-hemispheric interfer ence effects caused by activation of sensory areas in bilateral hemisp heres. Activated areas in the primary and second sensory cortices (SI and SII) in each hemisphere following bilateral stimulation were clear ly identified by estimation of the double best-fitted equivalent curre nt dipoles (ECD) using the spherical head model, and the large inter-i ndividual differences were identified. SEFs following the right poster ior tibial nerve stimulation and those following the left stimulation were summated ('summated' waveform). The 'difference' waveform was ind uced by a subtraction of 'bilateral' waveforms from the 'summated' wav eform. Short-latency deflections showed no consistent changes between the 'summated' and 'bilateral' waveforms, but the long-latency deflect ion, the N100m-P100m, in the 'bilateral' waveform was significantly (P < 0.02) reduced in amplitude as compared with the 'summated' waveform . The differences were clearly identified in the 'difference' waveform , in which the main deflections, U100m-D100m, were found. The ECDs of the short-latency deflections were located in SI contralateral to the stimulated nerve, but the ECDs of the N100m-P100m were located in bila teral SII which are considered to receive ascending signals from the b ody bilaterally. Therefore, some inhibitory interactions might take pl ace in SII by receiving inputs from the body bilaterally. (C) 1997 Els evier Science Ireland Ltd.