Modulation of intracortical facilitatory circuits of the human primary motor cortex by digital nerve stimulation

Methods: We investigated the effect of electrical digit stimulation on 2 different intracortical facilitatory phenomena. Paired-pulse transcranial magnetic stimuli with different conditioning stimulus (CS) intensities were applied over the primary motor cortex (M1). Electromyographic recordings were made from the relaxed right abductor digiti minimi muscle. The effect of preceding sensory stimulation applied to the ipsilateral digit V on the conditioning magnetic stimulus was examined.

Results: Changing the CS intensity affected the influence of peripheral electrical stimulation on motor evoked potential (MEP) amplitudes evoked by a subthreshold CS followed by a suprathreshold test stimulus (TS) at an interstimulus interval (ISI) of 10 msec. In contrast, varying CS intensities in a paired pulse paradigm with a suprathreshold first and a subthreshold second stimulus at ISI of 1.5 msec had no effect on the MEP decrease induced by digit stimulation.

Conclusion: These findings suggest that appropriately timed peripheral electrical stimuli differentially modulate facilitatory interactions in the primary motor cortex. They further support the hypothesis that intracortical facilitation (ICF) and short-interval intracortical facilitation (SICF) are evoked through different mechanisms.