TRANSCRANIAL MAGNETIC STIMULATION REVEALS A HEMISPHERIC-ASYMMETRY CORRELATE OF INTERMANUAL DIFFERENCES IN MOTOR-PERFORMANCE

Hemispheric asymmetries in the threshold for eliciting motor evoked po tentials (MEPs) with transcranial magnetic stimulation (TMS) are assoc iated with hand preference. We posited that hemispheric asymmetries in TMS thresholds may be strongly correlated with some hand-differences in motor performance. MEPs result from the activation of neuronal netw orks targeting large cortical motoneurons. Thus, MEP thresholds might reflect physiological features of the corticospinal motor system. Cons idering the role of corticospinal pathways in the control of independe nt finger movement, we hypothesized that MEP thresholds would better p redict speed and dexterity than strength. In 30 right-handers and 30 l eft-handers, we correlated right and left hand-differences in the thre shold for eliciting MEPs with hand-differences in the performance of t hree manual tasks: finger-tapping speed, pegboard dexterity, and grip strength. Correlations of hand-differences in TMS thresholds with hand -differences in performance indicated that a lower TMS threshold for o ne hand is strongly associated with greater ability with that hand. Th e correlations of hand-differences in TMS thresholds with hand-differe nces in finger-tapping and pegboard dexterity were significantly large r than the correlation of hand-differences in TMS thresholds with hand -differences in grip strength. Our results indicate that hemispheric a symmetries in MEP thresholds may have functional significance related to basic parameters of movement. These results are consistent with the critical role of the corticospinal motor system in the control of ind ependent finger movement. Furthermore, they imply that asymmetry in th e corticospinal motor system may be an important substrate for asymmet ries in hand preference and performance. (C) 1997 Elsevier Science Ltd .