Hemispheric asymmetry of surround inhibition in the human motor system

Objective: Surround inhibition (SI) in the motor system is an essential mechanism for the selective execution of desired movements. To investigate the relationship between the efficiency of SI operation in the motor system and handedness, we performed a transcranial magnetic stimulation (TMS) study in 10 healthy, right-handed volunteers. Methods: TMS was set to be triggered by self-initiated flexion of the index finger at different intervals ranging from 3 to 1000 ms. Average motor evoked potential (MEP) amplitudes obtained from self-triggered TMS were normalized to average MEPs of the control TMS at rest and expressed as a percentage. Normalized MEP amplitudes of the adductor digiti minimi (ADM) and the flexor digitorum superficialis (FDS) muscles were compared between the dominant and non-dominant hands. Results: During index finger flexion, MEP amplitudes of the ADM in the dominant hand were suppressed but not in the non-dominant hand, while MEP amplitudes of the FDS were comparably enhanced in both hands. F-wave amplitudes of ADM were comparably enhanced during index finger flexion in both hands. Conclusion: These results suggest that the functional operation of SI in the motor system is more efficient in the dominant hand than the non-dominant hand. More efficient SI in the dominant hand could lead to greater dexterity in the dominant hand. Significance: Hemispheric asymmetry of SI might be able to serve as a neurophysiological proxy for handedness. (C) 2009 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.