BACKGROUND
Transcranial magnetic stimulation (TMS), when applied over the primary motor cortex (M1) elicits a motor evoked potential (MEP) in electromyograms measured from peripheral muscles. MEP amplitude has often been observed to fluctuate trial by trial, even with a constant stimulus. Many factors cause MEP fluctuations in TMS. One of the primary factors is the weak stationarity and instability of cortical activity in the brain, from which we assumed MEP fluctuations originated. We hypothesized that MEP fluctuations must be suppressed if TMS is delivered to M1 at the time when several electroencephalogram (EEG) channels measured on the scalp have high similarity in the frequency domain.
OBJECTIVE
We developed a TMS triggering system for suppressing MEP fluctuations using EEG coherence analysis, which was performed to detect the EEG signal similarity between the two channels in the frequency domain.
METHODS
Seven healthy adults participated in the experiment to confirm whether the TMS trigger system works adequately, and the mean amplitude and coefficient of variation of the MEP were recorded and compared with the values in the control task. We also determined the experimental time under each condition and verified whether it was within the predicted time.
RESULTS
The coefficient of variation (CV) of MEP amplitude decreased in five out of seven subjects, and significant differences (P=.02) were confirmed in two of the subjects by performing an F-test. The CV of the experimental time required for each stimulus with threshold modification was less than that without threshold modification, and a significant difference (P<.001) was confirmed by performing an F-test.
CONCLUSIONS
We consequently found that MEP could be suppressed using the system developed in this study and that the TMS trigger system could also stabilize the experimental time by changing the triggering threshold automatically.
Development of a TMS Trigger System for Suppressing MEP Fluctuation Using EEG Coherence Analysis: Algorithm Development and Validation Study (Preprint)
