Subtle hemispheric asymmetry of motor cortical inhibitory tone

• Published in: Clinical neurophysiology 2004-02, Vol.115 (2), p.330-340
• Main Authors: Ilic, Tihomir V, Jung, Patrick, Ziemann, Ulf
• Format: Article
• Language: English
• Subjects: Adult; Biological and medical sciences; Differential Threshold; Dose-Response Relationship, Radiation; Electric Stimulation; Electromyography; Evoked Potentials, Motor - physiology; Female; Functional Laterality - physiology; Fundamental and applied biological sciences. Psychology; Hemispheric asymmetry; Humans; Magnetics; Male; Median Nerve - physiology; Motor control and motor pathways. Reflexes. Control centers of vegetative functions. Vestibular system and equilibration; Motor cortex; Motor Cortex - physiology; Motor evoked potential; Muscle Contraction; Muscle, Skeletal - physiology; Neural Inhibition - physiology; Reaction Time - physiology; Short-interval intracortical inhibition; Time Factors; Transcranial magnetic stimulation; Vertebrates: nervous system and sense organs
• ISSN: 1388-2457; 1872-8952
• DOI: 10.1016/j.clinph.2003.09.017

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 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.