Dry andWater-Based EEG Electrodes in SSVEP-Based BCI Applications - Biomedical Engineering Systems and Technologies - page 5

on the accuracy and ITR of the SSVEP BCI system. Furthermore, the impact of variety
of users, having a different hair length and type on the signal quality is often omitted.
In this paper we explore how headset setups using dry contact electrodes, water-based
electrodes (replicated setup from [41]), and conductive gel electrodes compare to each
other considering above mentioned aspects. We also evaluate the convenience levels for
the end users, as well as the time required for preparing participants for the experiment.
We believe that robust BCI systems with dry and water-based electrode solutions
would greatly simplify the usage, increase acceptance by users in certain clinical ap-
plications, and enable wider adoption of BCIs in consumer applications. Therefore, we
emphasize the importance of a practical EEG signal acquisition system which does not
require expert assistance, can be setup and removed by the user himself in a short pe-
riod of time, and is designed to be ergonomic, convenient, unobtrusive, and comfortable
during the measurement process.

3 Materials and Methods

In this section we present the brain signal acquisition technology consisting of an EEG
signal amplifier and three different electrode setups, followed by presenting the details
of the study design, data processing, and evaluation methods.

3.1 Amplifier Technology and Data Acquisition

The EEG data was recorded using the Twente Medical Systems International (TMSi)
Porti system with 24 EEG channels. The Porti uses bipolar amplifier technology that
amplifies the difference of the two inputs (so-called instrumentation amplifier technol-
ogy) with a gain of 20 and includes a common mode rejection in the second stage of
amplification. This technology prevents the issues caused by different gain in operation
amplifiers and amplifies the input signal against the average reference of the incoming
signals, i.e., the common mode signal. The common mode range is

− 2 Vto 2 V, a n d t h e

common mode rejection ratio is higher than 100 db.
The Porti is used in a battery powered mode and it was connected to a PC via an opti-
cal cable. The highest possible sample rate of 2 kHz was used (bit rate of 7 . 168 Mbit/s).
As the Porti system requirements include the usage of shielded cables for EEG elec-
trodes and ground electrode with low impedance ( < 1 k Ω ), we used shielded cables for
all electrodes and gel electrode as a ground for all three setups.

3.2 EEG Setups with Three Different Electrode Types

For all three setups we used 8 electrodes positioned at the occipital and parietal sites
where the SSVEP exhibits the strongest response. The 8 electrode locations selected
were O1, O2, Oz, PO3, PO4, POz, P1, and P2, according to the International 10-20
System. For all setups (except two configurations discussed in Section 5) conductive
gel ground electrode was positioned at the participants right collar bone.
The dry electrode setup is constructed using 8 commercially available sintered
Ag/AgCl ring electrodes (with 10 mm outer and 5 mm inner diameter) that have twelve