A model for removing transcranial current stimulation artifacts in concurrently measured EEG

Transcranial current stimulation (tCS) is a non-invasive brain stimulation technique that has shown promise for studying and improving brain function. It can be applied with low-amplitude direct (tDCS), alternating (tACS) or random noise (tRNS) currents. EEG, with its high temporal resolution, portability, and affordability, offers great advantages in investigating the effects of tCS on brain activity. However, concurrent EEG acquisition and tCS stimulation suffers from the drawback that injected current induces significant artifacts on simultaneously acquired EEG. Furthermore, stimulus-current-induced artifacts in measured voltages have powers that are large compared to that of EEG, in the frequency range of interest for EEG analysis. While simple high-pass filtering of the EEG would eliminate artifacts from tDCS, it is not suitable when stimulating with frequencies in the range of significant EEG activity (1-40 Hz). This occurs both in low-frequency tACS/tRNS and in high-pass filtered tRNS, as even in the latter case substantial power will remain at EEG frequencies. In such cases, attenuating tRNS artifacts in EEG requires a more comprehensive model, such as the one we present here.