PHYSIOLOGICAL MOTOR ASYMMETRY IN HUMAN HANDEDNESS - EVIDENCE FROM TRANSCRANIAL MAGNETIC STIMULATION

We hypothesized that human handedness might be associated with measura ble differences in the excitability of the motor system. We compared t he thresholds for electromyographic activation of the left and right a bductor pollicis brevis (APB) and biceps muscles in 30 left-handers an d 30 right-handers, by varying the direction of a brief monophasic pul se in a circular electromagnetic coil centered over the vertex of the scalp. In right-handers, we found that the threshold for activation of muscles in the right arm was lower than the threshold for activation of corresponding muscles in the left arm. In left-handers, the reverse was true. Threshold asymmetry was influenced significantly by the con sistency with which each subject used the writing hand to perform othe r motor tasks, and was not significant between non-consistent left-han ders and right-handers. Our results indicate that human handedness, an d in particular, consistency of hand preference, are associated with l ateralized differences in the excitability of motor system projections activated by transcranial magnetic stimulation. Our findings might re flect physiological differences in corticospinal tract function or cor tical motor representation.