Modulation of intracortical neuronal circuits in human hand motor area by digit stimulation

We investigated the changes in intracortical neuronal circuits of the hand motor cortex following sensory stimulation of the fingers in 11 healthy subjects. Motor evoked potentials (MEPs) were recorded from intrinsic hand muscles (right first dorsal interosseous and abductor digiti minimi muscles). Electrical stimulation was applied to a digit near (homotopic) or distant (heterotopic stimulation) from each muscle. The right index or little finger was stimulated electrically, followed by single- or paired-pulse transcranial magnetic stimulation (TMS) at an interval of 25, 200, 600, 1,000 or 1,400 ms. Paired-pulse TMS was applied with interstimuli intervals of 2 ms or 12 ms and was expected to stimulate inhibitory or facilitatory intracortical circuits, respectively. MEPs induced by single-pulse TMS were significantly suppressed 200, 600, and 1,000 ms after heterotopic and homotopic stimuli. Intracortical facilitation was significantly enhanced only after homotopic stimuli and such enhancement was maximal 200 ms after digit stimulation. Intracortical inhibition was slightly weakened after homotopic stimulation but this effect did not reach statistical significance (P=0.25). Our results show that sensory feedback can modify intracortical and corticospinal motor excitability and that intracortical facilitation can be enhanced in a topographic-specific way especially at long latencies. These findings suggest that indirect pathways, probably through somatosensory cortex and other areas, enhance intracortical motor excitability in a somatotopically organized manner.