A skin capacitor model for the TMS-induced artifact in EEG

Using transcranial magnetic stimulation (TMS) in combination with electroencephalography (EEG) permits the measurement of the brain's global response to localized and abrupt stimulations. However, TMS induces large artifacts in the EEG recordings which are unrelated to brain activity and obscure the response of the brain. These artifacts can be removed algorithmically, and various methods have been suggested. Furthermore, several skin preparation methods have been demonstrated to reduce the artifacts. However, the lack of a theoretical model for the physical processes underlying the artifacts and their dynamics hampers an assessment of the performance of these algorithms and an evaluation of their validity.

We find that, contrary to expectation, the decay of the electrode voltage after the TMS pulse follows a power law in time rather than an exponential. Based on this observation, we re-examine the role of the skin in creating the artifacts. We find that the artifact originates from a dual effect: The skin creates a first power law and this is coupled with the effect of the reference electrode that changes the order of the power law.

We present a theoretical model for diffusion of the accumulated charge from the high electric fields of the TMS in the skin. This model reproduces the artifact precisely, including the many artifact shapes that are seen on the different electrodes. Based on this model, a removal method is derived, which exposes the full EEG from the brain, as validated by reconstructing 50Hz signals that are the same magnitude as the brain signals. Our insight of the physical process allows to accurately access TMS evoked potentials after 1.5 milliseconds following the TMS pulse.