Techniques of EEG Recording and Preprocessing - Quantitative EEG Analysis Methods and Clinical Applications - page 29

scalp preparation is required to achieve EEG recordings of adequate quality. How-
ever, if low impedances are required in order to optimize the data SNR, then further
scalp preparation with a blunted needle is required, thus largely abolishing the time
advantage obtained when using this electrode type in high impedance mode. How-
ever, although this effectively limits the benefit of using this methodology to elimi-
nate the need to apply gel to the subject's hair, this advantage should not be
underestimated at least as a motivational factor in today's multichannel recordings.
Figures 2.3(a, b) show typical ring-shaped (EasyCap GmbH, Herrsching, Ger-
many) and closed (hat-shaped) Ag/AgCl electrodes that are commonly used today.
Another way of dealing with the impedance and noise problems inherent in clas-
sical passive electrode schemes is to use active electrodes (Figure 2.4). Active in this
context means that the electrodes themselves contain active electronics circuitry that
serve the purpose of treating the incoming high-impedance scalp electrical signal in
such a way that three main goals are achieved: (1) stable operation for a much
broader range of scalp impedances, (2) tolerance against a wider range of impedance
differences across the scalp, and (3) enhanced noise immunity along the cable path
toward the amplifier. The electronics principle by which these three goals are
achieved is known as impedance conversion. In its simplest form, impedance conver-
sion refers to a circuit consisting of an operational amplifier that is effectively set for

(a) (b)

(c) (d)

Figure 2.4 Examples of two commonly used active Ag/AgCl electrodes. (a) Upper side of an
EasyCap Active electronics circuit board with operational amplifier. (b) EasyCap Active after comple-
tion with Ag/AgCl Pellet and integrated electrode holder (EasyCap, Herrsching, Germany). (c) Upper
and (d) lower side views of the actiCap electrode with the LED-based onboard impedance check acti-
vated (Brain Products, Gilching, Germany).