Transcranial magnetic stimulation reveals asymmetrical efficacy of intracortical circuits in primary motor cortex.

The efficacy of inhibitory and excitatory intracortical circuits acting on the representation of an intrinsic hand muscle in the primary motor cortex of both hemispheres was measured with paired transcranial magnetic stimuli in right-handed subjects. Both intracortical inhibition (measured with an interstimulus interval of 3 ms) and intracortical facilitation (measured with an interstimulus interval of 16 ms) developed more rapidly with increasing conditioning stimulus intensity in the dominant than the non-dominant hand. We conclude that the intracortical circuits in the primary motor cortex are more potent in the dominant than the non-dominant hemisphere, and hypothesize that this difference is a factor in the asymmetrical dexterity associated with hand preference.