6.1 Signals of the Brain

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Fig. 6.1: EEG signals usually have no clear structure and thus belong to the non-deterministic sig-

nals. Nevertheless, a rhythm is recognizable in the signals.

electroencephalography (EEG). Because of the large number of cells involved and

their different temporal excitation, the EEG signal usually does not have a clear struc-

ture (cf. Figure 6.1). Thus, it belongs to the non-deterministic signals. However, the

EEG is underpinned by a rhythmic pattern, which depends among other things on the

mental alertness of the subject. In clinical diagnostics, this rhythm is called a wave².

If the frequency of the wave is in the range from 0 to 4 Hz, it is called a delta wave

(typical for dreamless deep sleep), from 4 to 8 Hz as theta wave (light sleep phase),

from 8 to 13 Hz as alpha wave (e.g. relaxed waking state), at 13 to 30 Hz as beta wave

(medium concentration, REM sleep) and above 30 Hz as gamma wave (strong concen-

tration). In Figure 6.2, EEG waveforms at different states of wakefulness are shown.

For the determination of the frequency distribution, a Fourier transformation of

the EEG time signal is performed. Figure 6.3 shows the spectrum of an EEG with a max-

imum at 11 Hz, suggesting alpha waves in the EEG signal. The distinction of different

states in the EEG spectrum usually requires a high frequency resolution. For example,

if a frequency resolution of 1 Hz is required, according to the uncertainty principle of

communications engineering, an EEG sequence of at least 1 s duration is necessary³.

The EEG electrodes have basically the same structure as ECG electrodes, but are

mostly smaller in diameter. The arrangement of the electrodes on the skull follows

a specific scheme. With the internationally standardized 0-20 System, 21 electrodes

are placed on meridians between the root of the nose and the lower edge of the pos-

terior skull bone at intervals of 10 % resp. 20 % of the meridian lengths. Figure 6.4

shows in side and top view the electrode arrangement of the 10-20 system. A meas-

urement curve results from the voltage difference between a measurement electrode

2 This is not a wave in the physical sense.

3 The uncertainty principle of communications engineering states that the product of time and fre-

quency resolution is greater than or equal to one: ∆t ∆f ≥1.