Techniques of EEG Recording and Preprocessing - Quantitative EEG Analysis Methods and Clinical Applications - page 27

The most commonly used electrode type consists of bonded (sintered) Ag/AgCl,
which quickly establishes and then maintains consistent and stable electrochemical
potentials against biological tissues, together with low dc offset variability. More-
over, Ag/AgCl electrodes are free from potential allergenic compounds and have
excellent long-term electrical stability.
Figure 2.3 shows some common examples of passive EEG electrodes. The term
passive EEG electrode implies that the electrode itself is not electrically active but
instead functions as a passive metal sensor that establishes electrochemical contact
to the scalp via the electrolyte used. This sensor then converts the changes in
charged ion concentrations on the scalp into an electrical current that is transmitted
along the electrode cable and is then measured in the biopotential instrumentation
amplifier.
Traditionally, closed (hat-shaped) electrode forms were used in EEG recordings
where a low number of individual electrode positions were used and the scalp was
abraded before electrode application. Typically, an electrolyte together with collo-
dion has been used to glue the electrode firmly to the scalp, for example, for
long-term recordings. However, with the high density and vastly multichannel
recordings employed today, placing individual electrodes is neither feasible, nor
would exact placement of large quantities of electrodes be practically possible.
Therefore, the use of electrode caps has become standard. These caps have electrode
holders fixed to the textile fabric and thus establish the approximate electrode posi-
tions without need for single electrode position measurements. Scalp preparation is

(a) (b)

(c)

(d)

Figure 2.3 Examples of commonly used passive Ag/AgCl electrodes. (a) Ring-shaped Ag/AgCl elec-
trode in electrode holder, granting easy access to the scalp for skin preparation (EasyCap, Herrsching,
Germany). (b) Classical “hat-shaped” electrode typically used for individually placed EEG electrode
derivations. (c) QuikCell electrode, cut open to reveal the cellulose sponge element used with an elec-
trolytic solution instead of a gel-based electrolyte (Compumedics, El Paso, Texas). (d) HydroCel elec-
trodes in a Geodesic Sensor Net (EGI, Eugene, Oregon).