Techniques of EEG Recording and Preprocessing - Quantitative EEG Analysis Methods and Clinical Applications - page 32

The most desired property of any EEG amplifier is that it amplifies the EEG sig-
nal and disregards or attenuates any undesired signal influences. Three different sig-
nal components have to be dealt with by an EEG amplifier: biological signals,
electrode offset signals, and mains noise signals. The EEG biosignals are the
summed potentials measured at a scalp electrode and consist of the cortical and, to a
small extent, subcortical activity. However, this signal is invariably compromised by
endogenous and exogenous artifacts, such as scalp and sweat potentials, eye move-
ment, and other EMG artifacts as well as artifacts related to the stimulus presenta-
tion, for example, electrical pain stimulation spikes. Thus, while the electrocortical
signal itself typically only has an amplitude range of approximately ± 150 μ V, the

total signal referred to here can have an amplitude range of 2 mV.

The dc offset component is inherent to the signal measurement with metallic
electrodes of any type. It fluctuates over time and, depending on the type of electrode
used, can reach large values of several hundred millivolts. High temporal dc offset
stability with low offset values of typically less than 100 mV is a common feature of
high-quality Ag/AgCl electrodes, which makes these the favored electrodes in EEG
research.
The mains noise is another, more obvious noise component consisting of sinu-
soidal artifacts at the mains frequency (50 or 60 Hz). The prevalence of mains noise
in the recording environment depends largely on the presence of mains powered
electrical devices in the vicinity of the recording equipment. Also, the type of equip-
ment present has a major influence on the magnitude of mains noise, with devices
containing electrical motors such as pumps, razors, and hair driers being particu-
larly “good” emitters of such noise. Mains noise is capacitively coupled into the
cables of the EEG electrodes and as such has a more profound influence on the signal
measured with high electrode impedances. Also, for the same reason applied to
capacitive coupling, keeping all electrodes together in a ribbon or bundle of cables
will largely reduce mains noise. Mains noise can be further reduced by operating the
EEG amplifier with (rechargeable) batteries.
Another source of this noise lies within the amplifier system itself. For patient
safety reasons, the subject has to be kept “floating” with regard to the mains and the
earth ground. However, because this would make the difference between the body
potential and amplifier potential arbitrary up to the level of the full mains voltage,
the body of the subject is typically connected to the patient ground. In this way, the
potential difference between the body and the amplifier inputs with reference to
mains is kept in a range of typically less than 100 mV. The mains noise signal is pres-
ent at all inputs (channels) of the amplifier and is therefore oftentimes called com-
mon mode noise. The common mode signal can also effectively be reduced using a
concept called active shielding , as discussed later in this chapter.
If we look at the summed potentials resulting from worst case Biosignal Off-
set Mains voltages (2 mV

±

202 mV), it becomes clear that such
levels would be beyond the digitization levels of most modern amplifiers and such a
signal could only be amplified by a factor of 25 before an EEG system built from
operational amplifiers powered with

+ 100 mV + 100 mV =

5V would saturate. However, modern EEG
amplifiers are built with multichannel instrumentation amplifiers, which are
designed to amplify only the biosignal portion at the gain set, while passing the off-
set voltages through unamplified and at the same time cancelling the mains noise.

±