Novel TMS-EEG indexes to investigate interhemispheric dynamics in humans

Objective: To validate two indexes of interhemispheric signal propagation (ISP) and balance (IHB) by combining transcranial magnetic stimulation (TMS) and electroencephalography (EEG). Methods: We used TMS-EEG to non-invasively stimulate the two hemispheres of 50 healthy volunteers and measured interhemispheric dynamics in terms of ISP and IHB. We repeated our evaluation after three weeks to assess the reliability of our indexes. We also tested whether our TMS-EEG measures were correlated with traditional interhemispheric inhibition (IHI), as measured with motor-evoked potentials (MEPs). Results: Our main results showed that ISP and IHB (1) have a high reproducibility among all the participants tested; (2) have a high test-retest reliability (3) are linearly correlated with IHI, as measured with MEPs. Conclusions: The main contribution of this study lies in the proposal of new TMS-EEG cortical measures of interhemispheric dynamics and in their validation in terms of intra- and inter-subject reliability. We also provide the first demonstration of the correlation between ISP and IHI. Significance: Our results are relevant for the investigation of interhemispheric dynamics in clinical populations where MEPs are not reliable.