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. 9x9 intensity conditions) ranging from 60 to 140% of resting motor threshold (RMT) were tested. The interaction between S1 and S2 was expressed by MEP(S1+S2)/(MEP(S1)+MEP(S2))*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.
Ilic
Tihomir V
TV
Clinic of Neurology, J.W. Goethe-University Frankfurt, Theodor-Stern-Kai 7, D-60590 Frankfurt am Main, Germany.
Jung
Patrick
P
Ziemann
Ulf
U
D000328:Adult; D004056:Differential Threshold; D004307:Dose-Response Relationship, Radiation; D004558:Electric Stimulation; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D007839:Functional Laterality / Q000502:physiology*; D006801:Humans; D008280:Magnetics; D008297:Male; D008475:Median Nerve / Q000502:physiology; D009044:Motor Cortex / Q000502:physiology*; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000502:physiology; D009433:Neural Inhibition / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D013997:Time Factors