TMS artifacts of a passive EEG system (g.tec) with ring electrodes.

<p>Traces were recorded from nine electrodes (F2, F5, F6, C3, C4, CZ, CP5, P5, POz) at 38,4 kHz on a single subject. TMS was applied at time 0. Stimulation sites were C3 (red traces, 4 trials), CPz (green, 3 trials), and CP2 (blue, 3 trials). (A) The TMS pulse artifact is well-resolved due to the high acquisition rate and has a duration of 0.39 ms. Note the short-time artifact dynamics visible in the green traces, which is different from what can be seen in the Biosemi artifacts. (B, C) The skin-capacitor discharge artifacts for a short time (B) and long time (C). (D) On a log-log scale, the late-stage decay of all artifacts exhibits a power law of order 2 like in the Biosemi EEG system. (E, F, G) Single trials with reconstructed artifact fits overlaid on the artefactual data (gray) for each stimulation site as indicated by color. (H) Log-log plot of the artifact fits for all trials overlaid on the data. The first 15 samples corresponding to 0.39 ms containing the TMS pulse were omitted. For each stimulation site the starting time point for the fit is selected as the first point at which all the artifacts in all electrodes already decay faster than a power law of order 1. This is actually the place where a power law of order 1 (magenta) is tangential to the slowest decaying artifact. These time points are marked by vertical lines that correspond to each stimulation site (for CP2 0.31 ms, C3 at 0.63 ms, CPz at 3.12 ms following the end of the TMS pulse). (I, J, K) Reconstructed data by subtraction of the artifact fits. Note that the noise in the form of spikes (arrows in I, J; also visible in E, F), possibly from the TMS stimulator, is reconstructed without distortion.