Signal Quality in Dry Electrode EEG and the Relation to Skin-electrode Contact Impedance Magnitude

Current EEG research approaches are focusing on developing new dry electrode EEG (electroencephalogram) systems providing a high enough signal quality for a wide range of applications. This study proposes several parameters for evaluating signal quality of dry electrodes and relates the results to skin-electrode contact impedance magnitude values. The EEG recordings of a Ag/AgCl pinned electrode and a flexible polymer pinned electrode are evaluated through a comparison to conductive gel electrode recordings. The experimental setup was made up of two EEG acquisition systems connected in parallel. The protocol included open eyes, closed eyes and steady-state visually evoked potentials (SSVEP) sessions in both seated and walking in place conditions. The parameters used for evaluation were signal correlations, signal coherence and signal-to-noise ratios (SNRs). Results showed that the three proposed parameters provided equivalent outcome for signal quality estimation for the same recordings. There was no relation reported between the defined signal quality and the skin-electrode contact impedance in either dry or gel electrodes, although high impedance variations were present among subjects. However, larger impedance magnitude and impedance magnitude variations, and lower signal quality is observed for dry electrodes compared to gel ones.