Biomedical Engineering Reference
In-Depth Information
The abrasion process, as well as the usage of conductive gel (electrolyte) makes
the whole EEG setup inconvenient for practical applications, especially in consumer
settings. The application of electrolyte and the electrodes, even when typical EEG caps
are used, requires expert assistance. The setup process is lengthy as it includes preparing
the skin, applying the gel, positioning the electrodes (or the cap) and ensuring that
the EEG signal quality level is acceptable. Additionally, the user (or expert) has to
remove the electrolyte and clean the user's head afterwards, and also clean and dry the
electrodes (and the cap) that were used. This also takes time and requires additional
effort. This paper aims at addressing these issues, focusing on two alternative solutions,
water-based and dry contact electrodes. It also addresses the issue of safe interaction in
BCI applications using the high frequency RVSi.
The paper is organized as follows. An overview of the alternative approaches for
EEG acquisition systems in BCI is given in the next section. Section 3 details the setups
for measuring EEG, using dry, water-based, and gel electrodes, as well as the methods
we used to evaluate the quality of the obtained signal in SSVEP BCI domain. Evalu-
ation of the three setups with respect to signal quality is presented in Section 4. Sec-
tion 5 addresses the potential practical application of water-based and dry electrodes in
SSVEP BCIs, focusing on the impact of stimuli duration, and looking into convenience
and comfort level of used setups. Results of the evaluation are discussed in Section 6.
Section 7 concludes the paper.
2
EEG Acquisition Systems in BCI: Overview
Few research laboratories realized the problem of cumbersome EEG acquisitions sys-
tems and engaged in developing more convenient techniques for acquiring brain signals.
The approaches range from developing hydrogel-based electrode [12] to numerous ver-
sions of dry electrodes. Dry electrode solutions include electrodes that are integrated
into the wearable material (contactless electrodes) or affixed on top of the scalp (insu-
lated electrodes) [13-18], electrodes that penetrate the outer layer of the skin [19-28],
and dry contact electrodes that exhibit galvanic contact to the skin without the usage of
additional electrolyte [29-32]. Contactless and insulated electrodes currently provide
insufficient signal quality [33], while due to skin damage they can cause, users that
use electrodes that penetrate the outer skin layers might be exposed to a higher risk of
infection and skin irritation [34]. Given the recent developments and positive evalua-
tion of dry contact electrode solutions [35-37], including the ones in the SSVEP-based
BCI domain [38-40], we consider dry contact electrodes as the desired technology for
convenient BCIs.
Addressing the SSVEP BCI field, in a recent publication [41], a successful BCI
application of electrodes that use cotton soaked in water to replace the conductive gel is
demonstrated. User investigation confirmed that so called 'water-based' electrodes are
preferred over gel-based ones, and that no significant performance drop is reported.
Despite the advances in convenient (SSVEP) BCIs, to the best of our knowledge,
none of the research publications have systematically characterized the performance of
the new BCI acquisition systems in terms of signal quality in different electrodes, and
the impact of signal quality, electrode selection, and classification algorithm parameters
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