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1 Division of Neurology, Krembil Neuroscience Centre and Toronto Western Research Institute, Toronto, Ontario, Canada; University of Toronto, Toronto, Ontario, Canada
* To whom correspondence should be addressed. E-mail: robert.chen{at}uhn.on.ca.
Motor cortex stimulation has both excitatory and inhibitory effects on ipsilateral muscles. Excitatory effects can be assessed by ipsilateral motor-evoked potentials (iMEPs). Inhibitory effects include an interruption of ipsilateral voluntary muscle activity known as the silent period (iSP), and a reduction in corticospinal excitability evoked by conditioning stimulation of the contralateral motor cortex (interhemispheric inhibition, IHI). Both iSP and IHI may be mediated by transcallosal pathways. Whether iSP and IHI represent the same phenomenon and their relationship to the contralateral corticospinal projection remain unclear. The neuronal population activated by transcranial magnetic stimulation (TMS) is highly dependent on the direction of the induced current in the brain. We examined the relationship between iMEP, iSP, IHI and the contralateral corticospinal system by examining the effects of different stimulus intensities and current directions. Surface EMG was recorded from both first dorsal interosseous (FDI) muscles. The iSP in the right FDI muscle was obtained by right motor cortex stimulation during voluntary muscle contraction. IHI was examined by conditioning stimulation of the right motor cortex followed by test stimulation of the left motor cortex at interstimulus intervals (ISIs) of 2 to 80 ms. The induced current directions tested in the right motor cortex were anterior medial (AM), posterior medial (PM), posterior lateral (PM) and anterior lateral (AL). Contralateral MEPs (cMEPs) had the lowest threshold with the AM direction and the shortest latency with the PM direction. iMEPs were present in 8 out of 10 subjects. Both iMEP and IHI did not show significant directional preference. iSP was observed in all subjects with the highest threshold for the AL direction and the longest duration for the AM direction. cMEP, iSP and IHI all increased with stimulus intensity up to about 75% stimulator output. Target muscle activation decreased IHI at 8 ms ISI but had little effect on IHI at 40 ms ISI. iSP and IHI at 8 ms ISI did not correlate at any stimulus intensities and current directions tested, and factor analysis showed that they are explained by different factors. However, active IHI at 40 ms ISI was explained by the same factor as iSP. The different directional preference for cMEP compared to iMEP and IHI suggest that these ipsilateral effects are mediated by populations of cortical neurons that are different from those activating the corticospinal neurons. iSP and IHI do not represent the same phenomenon and should be considered complementary measures of ipsilateral inhibition.
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