6.1 Signals of the Brain

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eyes open

eyes open

eyes open

eyes open

eyes open

eyes open

eyes

closed

eyes

closed

eyes

closed

time t / s

Fig. 6.5: EEG signal from the Berger experiment (top); the patient initially had his eyes closed and

opened them at approximately 25, 65, and 105 s for 10 s, respectively. The time-frequency spectrum

(below) shows the time course of the spectrum. In time periods with eyes closed, alpha waves occur

in the frequency range of 8–13 Hz occur with varying intensity (red, yellow, green regions in the

spectrum). If the eyes are closed these frequency components in the spectrum disappear again.

figure

plot (F1xx, (P1xx))

title('Power density spectrum of an EEG');

xlabel('f / Hz')

ylabel('Power density')

axis([0 50 0 2.5e4])

The results of the evaluation with Listing 6.1 are shown graphically in Figure 6.5. When

the patient is in a resting state with eyes closed, dominant frequencies in the range

between 8 and 13 Hz (alpha waves) occur in the time-frequency spectrum. If the eyes

are opened (at 25, 65, and 105 s for the duration of 10 s, respectively), a suppression

of alpha waves can be observed.

In addition to the evaluation of the EEG spectrum, analysis in the time domain is

also of high diagnostic importance. In epileptic seizures, additional spikes, which can

occur individually or in groups, often overlay the EEG signal. In evoked⁵ potentials, an

5 Lat. evocare = to bring about