Conductive Foam-surfaced Electrode for Capacitively-coupled EEG Measurement in Brain-Computer Interface

Abstract

Brain-computer interface (BCI) technologies have been intensively studied as a means to provide alternative communication tools that are entirely independent of neuromuscular activities. Current BCI technologies use electroencephalogram (EEG) acquisition methods that require unpleasant gel injections, impractical preparations, and clean-up procedures. The next generation of BCI technologies requires practical, user-friendly, nonintrusive EEG platforms that will facilitate the application of laboratory work in real-world settings. A capacitively-coupled electrode that does not require electrolytic gel or direct electrode-scalp contact is a potential alternative to the conventional wet electrode in future BCI systems. In this paper, a new conductive polymer foam surfaced electrode was proposed for use as a capacitively-coupled EEG electrode for nonintrusive EEG measurements in out-of-hospital environments. The current capacitively-coupled electrode has a rigid surface that produces an undefined contact area due to its stiffness, which renders it unable to conform to head curvature and locally isolates hairs between the electrode surface and scalp skin, thereby complicating EEG measurement through hair. This issue was overcome by applying a conductive polymer foam to the high-input-impedance active electrode surface to provide a cushioning effect. This enabled EEG measurement through hair without any requirement for conductive contact with bare scalp skin. Experimental results showed that the new electrode provided lower electrode–skin impedance and higher voltage gains, signal to-noise ratios, signal-to-error ratios, and correlation coefficients between EEGs measured by capacitively-coupled and standard resistively-coupled methods when compared to a conventional capacitively-coupled electrode. In addition, the new electrode could measure EEG signals, while the conventional capacitively-coupled electrode could not. The expectation is that the new electrode presented here can be easily installed in a hat or helmet to create a nonintrusive wearable EEG apparatus that does not make users look strange for real-world EEG applications. This paper also presents results from five subjects who exhibited visual or auditory steady-state responses according to BCI using these new capacitively-coupled electrodes. The steady-state visual evoked potential (SSVEP) spelling system and the auditory steady-state response (ASSR) binary decision system were employed. Offline tests demonstrated classification accuracies high enough to be used in a BCI system (95.2% for SSVEP BCI (6s), 82.6% for ASSR BCI (14s)) with analysis time slightly longer than that reported in the literature when wet electrodes were employed with the same BCI system. Subjects performed online BCI under the SSVEP paradigm in copy spelling mode and under the ASSR paradigm in selective attention mode with a mean information transfer rate (ITR) of 17.78±2.08 and 0.7±0.24 bpm, respectively. The results of these experiments demonstrate the feasibility of using the proposed capacitively-coupled EEG electrode in BCI systems. This electrode may become a flexible and non-intrusive tool fit for various applications in the next generation of BCI technologies.