Human handedness and asymmetry of the motor cortical silent period

We performed transcranial magnetic stimulation of the motor cortex in 22 le ft-handed and 25 right-handed subjects during active contraction of a small hand muscle. Motor evoked potentials had the same latency, amplitude and t hreshold on both sides of the body, whilst the silent period duration was s horter in the dominant hand. Silent periods elicited by nerve and brainstem stimulation were the same in both hands. Since the latter part of the cort ical silent period is due mainly to withdrawal of corticospinal input to sp inal motoneurones, we speculate that the results are compatible with the su ggestion that tonic contractions of the non-dominant hand are associated wi th a greater involvement of the corticospinal tract than those of the domin ant hand. It also seems likely that there is an asymmetry in the excitabili ty of cortical inhibitory mechanisms with those responsible for the cortica l silent period being less excitable in the dominant motor cortex.