Intra and interhemispheric modulation of motor and somatosensory cortices using cTBS: a TMS/EEG study

Introduction Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive technique widely used to probe the function of a given brain structure. It also has promising therapeutic applications for the treatment of various neuropsychiatric disorders. However, the after-effects of rTMS are highly variable across individuals. Recently, Hamada et al. (2013) found that the effect of continuous theta burst stimulation (cTBS) on M1 could be predicted by the latency of the motor-evoked potentials (MEPs) recorded before applying cTBS. This suggests that inter-individual variability of the effects of rTMS could be driven by differences in the interneuronal networks preferentially activated by TMS. Methods We investigated the effect of cTBS delivered over M1 on the ipsilateral and contralateral M1 and primary somatosensory cortex (S1) using motor-evoked potentials (MEPs), somatosensory-evoked brain potentials (SEP) and TMS-evoked brain potentials (TEP) recorded from both hemispheres before and after cTBS. Results Our results confirm that the variable effect of cTBS can be predicted by the latency of the MEPs recorded before applying cTBS. Short MEP latencies were associated with an enhancement of the MEPs elicited by stimulation of the ipsilateral M1 whereas late MEP latencies were associated with a reduction of MEP amplitude. In contrast, there was a positive correlation between MEP latency and the change in the amplitude of the MEPs elicited by stimulation of the contralateral hemisphere. A similar relationship was observed between MEP latency and the effect of cTBS on the magnitude of the N100 wave of TEPs. Finally, there was a group-level effect of cTBS on the magnitude of the N20 and P100 waves of SEPs, which, after cTBS, were decreased over the ipsilateral hemisphere and increased over the contralateral hemisphere. Discussion The reverse relationship between MEP latency and the effects of cTBS on the ipsilateral and contralateral hemisphere is compatible with interhemispheric inhibitory interactions