Application to biomedical signals - Practical Biomedical Signal Analysis Using MATLAB - page 107

different spectral content. In lower vertebrates electric brain activity is also observed,
but it lacks the rhythmical behavior found in higher vertebrates.

4.1.3 EEG measurement, electrode systems
EEG is usually registered by means of electrodes placed on the scalp. They can
be secured by an adhesive or embedded in a special snug cap. A block diagram of
the recording setup is shown in Figure 4.3. The first elements of the system are the
electrodes and the differential amplifier. The quality of the measurement depends
to a large extent on the ratio between the impedance of the electrodes and an input
impedance of the amplifier. This requirement is met by modern amplifiers which
have an input impedance up to 10 12 Ω. In such a case the standard requirement that
the electrode resistance be less than 5 kΩ may be slightly relaxed.
Prior to sampling, low pass anti-aliasing filters (Sect. 1.2.1.1) are used; high pass
filters are applied in order to eliminate artifacts of lowest frequencies. EEG is usu-
ally digitized by an analog-to-digital converter (ADC). The digitization of the signal
introduces the quantization error (Sect.1.2.2), which depends on the covered volt-
age range and the number of the ADC's bits (for modern apparatus up to 24 bits).
The sampling frequency ranges from 100 Hz for spontaneous EEG and several hun-
dred Hz for ERP to several kHz for recording short latency ERP and for intracranial
activity. Knowledge of the exact positions of electrodes is very important for both

High-pass
filter

Low-pass
filter

Electrodes

Analog to
Digital
Converter

recording
system

Amplifier

FIGURE 4.3: Block diagram of recording setup.

interpretation of a single recording as well as for comparison of results obtained
for a group of subjects; hence the need for standardization. The traditional 10-20
electrode system gives positions of 19 EEG electrodes (and two electrodes placed
on earlobes A1/A2) related to specific anatomic landmarks, such that 10-20% of
the distance between them is used as the electrode spatial interval. The first part of
the derivation's name indexes the array's row—from the front of head: Fp, F, C, P,
and O ( Figure 4.4) . In the extended 10-20 system, electrode sites halfway between
those defined by the standard 10-20 system were introduced. Nowadays 100 or even
200 electrode systems are used. The number of electrodes providing adequate space
sampling is 128, since inter-electrode distance should be around 20 mm to prevent

spatial aliasing [Nunez and Srinivasan, 2006].
EEG is a measure of potential difference; in the referential (or unipolar) setup it
is measured relative to the same electrode for all derivations. There is no universal
consent regarding the best position of the reference electrode. Since currents com-

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