Correlated amplitudes of potentials evoked in homologous muscles by magnetic stimulation reveal positive covariation of corticospinal output.

It is widely held that in human, fibres of the corpus callosum mediate inter-hemispheric inhibition - deemed necessary to prevent a bilateral cerebrum from generating simultaneous and potentially conflicting outputs. Ostensible support comes from an electrophysiological phenomenon whereby the mean magnitude of 'test' motor evoked potentials (MEPs) obtained in response to magnetic stimuli delivered over the contralateral motor cortex is diminished when initial 'conditioning' magnetic stimuli have been applied 6-15 ms previously to the opposite motor cortex. A contrary view is that this phenomenon masks, rather than reveals, normal physiological processes. An alternative hypothesis is that cortical motor centres giving rise to efferent projections onto motoneurons innervating homologous muscles conduct reciprocal shaping of excitation. This hypothesis was examined in a large sample (205 participants) by correlating the amplitude of MEPs elicited by a conditioning stimulus (CS) with the amplitude of those elicited 10 ms later by a test stimulus (TS). The magnitudes of responses to the CS and TS were positively correlated. This remained the case following statistical compensation for an observed covariation of low amplitude fluctuations in the background (<2 µV root mean squared) electromyographic activity recorded in the (homologous) target muscles prior to stimulation. Although the coefficients representing the magnitude of association between responses to the CS and TS are small (rho < 0.20), they are reliable. These findings support the hypothesis that there is positive covariation in the excitability of corticospinal projections from the two cerebral hemispheres to homologous muscles of the upper limb.