Interhemispheric inhibition (IHI) refers to the neurophysiological mechanism in which one hemisphere of the brain inhibits the opposite hemisphere. IHI can be studied by transcranial magnetic stimulation using a conditioning-test paradigm. We investigated IHI from 5 motor related cortical areas in the right hemisphere to the left primary motor cortex (M1). These areas are hand and face representations of M1, dorsal premotor cortex, somatosensory cortex, and dorsolateral prefrontal cortex. Test stimulus was delivered to the left M1 and conditioning stimulus (CS) was delivered to one of 5 motor related cortical areas in the right hemisphere. The time course of IHI, effects of different CS intensities and current directions on IHI were tested. Maximum IHI was found at interstimulus intervals of approximately 10 ms (short latency IHI, SIHI) and approximately 50 ms (long latency IHI, LIHI) for the motor related areas tested. LIHI could be elicited over a wide range of CS intensities, whereas SIHI required higher CS intensities. We conclude that there are 2 distinct phases of IHI from motor related cortical areas to the opposite M1 through the corpus callosum, and they are mediated by different neuronal populations.
cranial magnetic stimulation (TMS) to the primary motor cortex (M1) produces a series of corticospinal descending waves, with a direct (D) wave followed by several indirect (I) waves. TMS inducing posterioranterior (PA) current in the brain predominantly recruits the early I1-wave, whereas anterior-posterior (AP) directed current preferentially recruits the late I3-wave. However, it is not known whether I-waves elicited by different current directions are mediated by the same neuronal populations. We studied the neuronal mechanisms mediating I-waves by examining the influence of short-latency afferent inhibition (SAI) on various I-waves. SAI was tested with electrical median nerve stimulation at the wrist followed by TMS to the contralateral M1 at different current directions. Surface electromyograms and single motor units were recorded from the first dorsal interosseous muscle. SAI was weaker for the AP compared with that for the PA current direction. With increasing median nerve stimulation intensities, SAI increased for the PA direction but showed a U-shaped relationship for the AP direction. SAI produced more inhibition of late I-waves generated by PA than those generated by AP current direction. We conclude that late I-waves generated by PA and AP current directions are mediated by different neuronal mechanisms.
The beta and gamma frequency bands in the GPi are modulated by the preparation of self-initiated movements and the execution of self-initiated and externally triggered movements. The 5-18 Hz coherence at the bilateral GPi may be related to dystonia and its attenuation may facilitate voluntary movements.
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