Evoked EEG mapping during transcranial magnetic stimulation

The combination of transcranial magnetic stimulation (TMS) and electroencephalogram (EEG) is an effective method for investigating the cortical reactivity and the functional connectivity in the brain. This study focused on the measurement of evoked potential induced by TMS and obtaining the topography of the EEG over the whole head. In our past study, the electroencephalograph that removes artifact that has been caused by the magnetic stimulation was developed, and the brain wave evoked by TMS was measured. However, it is difficult either to measure the EEG over the whole head or to obtain the current distribution in the brain. In this study, the EEG over the whole head was able to be measured, and the EEG topography was obtained too. It is possible to measure the evoked EEG after a stimulation at 10 ms. In order to investigate the relationship between the stimulus strength and the evoked potential, the stimulus intensity was changed at 3 levels, motor threshold(MT), 90% of motor threshold, 80% of motor threshold and 70% of motor threshold. The result showed that there seemed to be a relationship between the stimulus intensity and the amplitude of evoked EEG. Topographies of the evoked EEG were obtained after a latency at 30 ms. We also observed the spreading of the EEG response to the contralateral hemisphere. It is possible to consider that the evoked signal by TMS to the motor area was conducted to contralateral hemisphere along the corpus callosum.