INTRACEREBRAL INTERACTIONS CAUSED BY BILATERAL MEDIAN NERVE-STIMULATION IN MAN - A MAGNETOENCEPHALOGRAPHIC STUDY

Somatosensory evoked magnetic fields (SEFs) following stimulation of t he median nerves bilaterally ('bilateral' waveform) were examined in n ormal subjects to determine the interference effects of activation of sensory areas in bilateral hemispheres. SEFs following right median ne rve stimulation and those following left median nerve stimulation were summated ('summated' waveform). A 'difference' waveform was induced b y subtraction of the 'bilateral' waveform from 'summated' waveform. Sh ort-latency deflections showed no consistent differences between the ' summated' and 'bilateral' waveforms, but the middle-latency deflection , N60m-P60m, in the 'bilateral' waveform was significantly (P < 0.01) smaller than that in the 'summated' waveform. The long-latency deflect ion, the N90m-P90m, in the 'bilateral' waveform was markedly (P < 0.00 1) reduced in amplitude as compared with the 'summated' waveform. The differences were clearly identified in the 'difference' waveform, in w hich the main deflections, U90m-D90m, were found in all subjects. Equi valent current dipoles (ECDs) of the short- and middle-latency deflect ions were located in the primary sensory cortex (SI) contralateral to the stimulated nerve, but ECDs of the N90m-P90m and U90m-D90m were loc ated in bilateral second sensory cortices (SII) which are considered t o receive ascending signals from bilateral sides of the body.