Dry andWater-Based EEG Electrodes in SSVEP-Based BCI Applications - Biomedical Engineering Systems and Technologies - page 2

Dry and Water-Based EEG Electrodes
in SSVEP-Based BCI Applications

Vojkan Mihajlovic 1 , Gary Garcia-Molina 1 , and Jan Peuscher 2

1 Philips Research, High Tech Campus 34, Eindhoven, The Netherlands
2 Twente Medical Systems International, Zutphenstraat 57, Oldenzaal, The Netherlands
{ vojkan.mihajlovic,gary.garcia } @philips.com,
[e-mail]

Abstract. This paper evaluates whether water-based and dry contact electrode
solutions can replace the gel ones in measuring electrical brain activity by the
electroencephalogram (EEG). The quality of the signals measured by three se-
tups (dry, water, and gel), each using 8 electrodes, is estimated for the case of
a brain-computer interface (BCI) based on steady state visual evoked potential
(SSVEP). Repetitive visual stimuli in the low (12 to 21Hz) and high (28 to 40Hz)
frequency ranges were applied. Six people, that had different hair length and type,
participated in the experiment. For people with shorter hair style the performance
of water-based and dry electrodes comes close to the gel ones in the optimal
setting. On average, the classification accuracy of 0 . 63 for dry and 0 . 88 for water-
based electrodes is achieved, compared to the 0 . 96 obtained for gel electrodes.
The theoretical maximum of the average information transfer rate across partici-
pants was 23 bpm for dry, 38 bpm for water-based and 67 bpm for gel electrodes.
Furthermore, the convenience level of all three setups was seen as comparable.
These results demonstrate that, having optimized headset and electrode design,
dry and water-based electrodes can replace gel ones in BCI applications where
lower communication speed is acceptable.

Keywords: Dry electrodes, Water-based electrodes, EEG, Signal quality,
Brain-computer interface, BCI, Steady state visual evoked potential, SSVEP.

1 Introduction

Brain computer interface (BCI) technology has not yet reached wider adoption except
for the few cases where it is used by severely impaired patients (for a recent review
see [1]). A number of research groups are trying to bring this technology to a more
advanced level, mainly focusing on the most convenient of brain sensing solutions - the
electroencephalogram (EEG) - which measures electrical activity of the brain.
Despite numerous advances, both in technological and ergonomic aspects, all three
predominant noninvasive BCI modalities, namely steady state visual evoked potential
(SSVEP), motor imagery, and P300 are still bound to laboratory settings and do not
show clear signs of being ready for wider commercialization in coming years. Among
the many problems that EEG-based BCI systems face, the most important ones include: