Subtle hemispheric asymmetry of motor cortical inhibitory tone

Objective: To test whether a novel paired transcranial magnetic stimulation (TMS) protocol (J Physiol 545.1 (2002) 153) detects hemispheric differences in motor cortical inhibition. Methods: Nine right-handers and 8 left-handers participated. Focal paired TMS was applied to the hand area of the dominant (M1-D) or non-dominant motor cortex (M1-ND). Motor evoked potentials (MEP) were recorded from the relaxed contralateral abductor digiti minimi. The first (S1) and second pulse (S2) were separated by 1.5 or 2.1 ms. Nine stimulus intensities of S1 and S2 (i.e. 9×9 intensity conditions) ranging from 60 to 140% of resting motor threshold (RMT) were tested. The interaction between S1 and S2 was expressed by MEPS1+S2/(MEPS1+MEPS2)*100%. Values below and above 100% indicate short-interval intracortical inhibition (SICI) and facilitation (SICF), respectively. Results: In right-handers, RMT was lower, SICI was present with fewer intensity conditions and the magnitude of SICI was less in M1-D than M1-ND. No hemispheric asymmetry was found for SICF. Left-handers showed no hemispheric difference for any of these measures. Conclusions: Findings suggest that, in right-handers, M1-D is controlled by less inhibitory tone than M1-ND. This may put the M1-D to an advantage for processes that are associated with a reduction of SICI, such as voluntary activation and use-dependent plasticity.