Modulation of short-latency intracortical inhibition in human primary motor cortex during synchronised versus syncopated finger movements.

Rhythmic movements are inherently more stable and easier to perform when they are synchronised with a periodic stimulus, as opposed to syncopated between the beats of a pacing stimulus. Although this behavioural phenomenon is well documented, its neurophysiological basis is poorly understood. In a first experiment, we demonstrated that all healthy subjects (N=8) performing index finger abduction in time with an auditory metronome exhibited transitions from syncopation to synchronisation when the metronome tempo was scaled from 0.8 to 2.0 Hz. Subjects' mean transition frequency was 1.7+/-0.2 Hz. In a second experiment, we used paired-pulse transcranial magnetic stimulation to examine short-latency intracortical inhibition (sICI) directed towards the first dorsal interosseous (FDI) muscle in healthy subjects (N=9) who made synchronised and syncopated phasic finger movements in time with metronome pacing of 1.0 Hz. Despite the equivalence between the patterns in terms of task performance and corticospinal excitability of FDI at this movement frequency, there was significantly greater sICI during syncopation than during synchronisation. From this result, we infer that the stability of movement patterns may be contingent upon excitability of inhibitory networks within primary motor cortex.