Reliable and Efficient TMS-EEG Using Ultra-Thin Active Electrodes

Transcranial magnetic stimulation combined with electroencephalography (TMS-EEG) enables direct assessment of cortical excitability and connectivity via TMS-evoked potentials (TEPs). While passive electrodes remain the gold standard, they require extensive preparation and are sensitive to impedance. Active electrodes offer practical advantages but are limited by increased height and susceptibility to decay artifacts requiring manual correction. We present a novel TMS-EEG setup with ultra-thin active electrodes (3 mm height), combined with hardware and software-based artifact suppression. In 10 healthy subjects, we performed simultaneous active and passive EEG recordings during TMS over the left primary motor cortex. We evaluated early (15–80 ms) and late (80–350 ms) TEPs using concordance correlation coefficients (CCC), linear mixed-effects modeling of amplitude variability, and convergence analysis across trial counts. Active and passive electrodes showed high signal consistency (median CCC: 0.97 for early, 0.96 for late TEPs). Within-type CCCs ranged from 0.91 to 0.97. TEP amplitudes did not differ significantly between electrode types, while distance from stimulation site affected amplitude. Reliable waveforms emerged quickly: mean CCC > 0.8 after 20 trials and > 0.9 after 30. Our results demonstrate the potential of active TMS-EEG as a reliable and efficient alternative to passive setups, overcoming key technical limitations that have previously restricted its broader adoption.