Electrical somatosensory stimulation modulates hand motor function in healthy humans

In healthy people, electrical somatosensory stimulation modulates excitability in contralateral cortical motor areas. The question whether this is associated with a change in motor performance is still under debate. The effect of electrical somatosensory stimulation on motor performance of the hand was investigated in 14 healthy right-handed subjects. Subjects performed index finger and hand tapping movements as well as reach-to-grasp movements towards small and large cubes with each hand prior to (baseline condition) and following 2-hour electrical somatosensory stimulation (trains of 5 pulses at 10 Hz with 1 ms duration delivered at 1 Hz with an intensity on average 60 % above the individual somatosensory threshold) of the (i) right median nerve, (ii) left median nerve, (iii) right tibial nerve (control stimulation) and (iv) left tibial nerve (control stimulation) on separate occasions at least one week apart. The order of sessions was counterbalanced across subjects. Somatosensory stimulation of the median nerves, but not of the tibial nerves, reduced the frequency and velocity of index finger and hand tapping movements performed with the stimulated hand, compared to baseline. In contrast, the kinematics of reach-to-grasp movements remained unaffected by somatosensory stimulation. The data suggest that somatosensory stimulation interferes with the processing of highly automated open-loop motor output at the stimulated limb, as reflected by tapping movements, but not with the processing of closed-loop motor performance, as reflected by reach-to-grasp movements.