  Transcranial magnetic stimulation combined with electroencephalography
(TMS-EEG) is widely used to study the reactivity and connectivity of brain
regions for clinical or research purposes. The electromagnetic pulse of the TMS
device generates at the instant of administration an artifact of large
amplitude and a duration up to tens of milliseconds that overlaps with brain
activity. Methods for TMS artifact correction have been developed to remove the
artifact and recover the underlying, immediate response of the cerebral cortex
to the magnetic stimulus. In this study, three such algorithms are evaluated.
Since there is no ground truth for the masked brain activity, pilot data formed
from the superposition of the isolated TMS artifact on the EEG brain activity
are used to evaluate the performance of the algorithms. Different scenarios of
TMS-EEG experiments are considered for the evaluation: TMS at resting state,
TMS inducing epileptiform discharges and TMS administered during epileptiform
discharges. We show that a proposed gap filling method is able to reproduce
qualitative characteristics and in many cases closely resemble the hidden EEG
signal. Finally, shortcomings of the TMS correction algorithms as well as the
pilot data approach are discussed.
