Excitability changes of hand motor area dependent on afferent inputs induced by different facilitation methods

Motor imagery (MI) and voluntary teeth clenching (VTC) induce excitability changes of hand motor area in primary motor cortex (M1). Additionally, M1 receives thalamocortical and corticocortical afferent inputs. However, relationships between afferent inputs and excitability changes induced by above-mentioned facilitation methods are not clear. Therefore, we investigated whether different effects of MI and VTC occur on early and late I-wave components using transcranial magnetic stimulation (TMS). TMS was applied to the left M1 by antero-medially directed current (to preferentially elicited early I-wave components) and by postero-laterally directed current (to preferentially elicited late ones). Motor evoked potentials (MEPs) were recorded from the right relaxed first dorsal interosseous (FDI) muscle. During MI, MEP amplitudes of late components were significantly larger than those of early ones, although both MEP amplitudes were enhanced. Surprisingly, during VTC, MEP amplitudes of early components were significantly enhanced, but those of late ones were rather depressed. That is, afferent inputs to M1 induced by MI enhanced both early and late I-wave components, but those by VTC have separate influences. Thus, it is assumed that early and late components of I-waves could be differently recruited depending on different afferent input pathways to M1.