Excitability changes in human corticospinal projections to muscles moving hand and fingers while viewing a reaching and grasping action.

Excitability of the H-reflex in the relaxed flexor digitorum superficialis (FDS) muscle was tested in five subjects observing a reaching and grasping action. The amplitude of the FDS H-reflex was modulated with a peak occurring during the hand-opening phase of the observed movement. When the H-reflex was facilitated by subliminal transcranial magnetic stimulation (TMS), the modulation was larger than for an unconditioned reflex of similar size. This suggests that the primary motor cortex excitability is modulated by action viewing and reasonably causes the motoneuronal excitability changes. Moreover, motor evoked potentials (MEPs) were elicited by supraliminal TMS in FDS, flexor carpi radialis (FCR) and first dorsal interosseus (FDI) when observing the same movement. MEP amplitude was modulated in FDS with the same time-course as the H-reflex, the peak excitability occurring during hand opening. In FDI, however, the maximal excitability occurred during finger closing while in FCR no correlation was found with the movement phases. Finally the EMG activity of FCR, FDS and FDI was recorded while the subjects were actually performing a grasping movement similar to the one observed. In all subjects and for each muscle there was a clear-cut correspondence between the time-course of the excitability modulation of MEPs and the temporal pattern of EMG recruitment. In conclusion, the present study suggests that 'motor resonance' subliminally activates the same motor pathways that would be overtly recruited in each observer when actually performing the observed movement, reproducing the personal strategy adopted in the same task.