Evaluation

Depending on the problem, different parameters are taken into account when evaluating the electroencephalogram. To characterize the EEG waves, first their frequency is determined. During periods of high stress on the neurons of the cerebrum, such as when solving a difficult mental exercise, the EEG can register waves with a frequency of 30-80 Hz (Hz=Hertz, unit of frequency, 1 Hz=1 wave per second).

These kinds of waves in electroencephalography are called gamma waves. So-called beta-waves have a frequency between 15-30 Hz and occur mainly when the eyes are open in the waking state. The relatively high frequency is caused by sensory impressions that are processed in the brain.

The wave type with the next lower frequency is the alpha waves. They are in the frequency range between 10-15 Hz and are registered by the electroencephalogram when the brain is awake but with closed eyes.Using the example of alpha waves, it is easy to see that the loss of sensory impressions, such as vision, leads directly to a reduction of the frequency in the EEG. If the eyes of the patient are closed and he is in a light sleep, theta waves occur.

They have a frequency of 5-10 Hz. The lowest frequency is reached during deep sleep with the so-called theta waves. Here only 3-5 waves per second (3-5 Hz) can be detected.

Electroencephalography is also an important component in the characterization of sleep stages. In addition to the wave types already mentioned, so-called sleep spindles also occur during sleep. These appear in the EEG as short high-frequency discharges with relatively high amplitude.

They occur primarily in sleep stage II. Also in this stage so-called k-complexes can be observed. A k-complex is a section in the EEG with a very high amplitude but low frequency and is probably related to a high synchronicity of thalamic nerve cells.

A last characteristic picture in the EEG are spike-and-wave-complexes. These high-frequency waves with high amplitude can be measured in the electroencephalogram during an epileptic seizure. The spike-and-wave complexes are due to a pathological (diseased) overactivity of certain nerve cells of individual brain regions during a seizure.

With the help of electroencephalography (EEG), an electroencephalogram is created on which the course and strength of the bioelectrical activity of the brain is recorded. This electroencephalogram contains waves that are evaluated according to certain frequency patterns (frequency bands), amplitude patterns, local activity patterns and their frequency of occurrence. Generally speaking, it is considered which curves are present, how fast they are, whether they are deformed and whether the curves have certain patterns.

Special computer-aided methods (e.g. spectral analysis) can also be used for evaluation. Particularly information-rich in the evaluation are the frequency bands, which can generally be divided into four categories:Delta-wave frequencies from 0.5 to 3 Hz: This frequency band can be observed especially in deep sleep and is characterized by slow and large amplitudes in the electroencephalogram. Theta-wave frequencies from 4 to 7 Hz: These frequencies occur during deep relaxation or while falling asleep.

Slow theta waves are normal in children and adolescents. In adults who are awake, the permanent occurrence of theta waves (and also delta waves) is a conspicuous finding. Alpha waves Frequencies between 8 to 13 Hz: These frequencies represent the basic rhythm of the biolelectric activity of the brain and appear in the electroencephalogram when the patient’s eyes are closed and the patient is at rest. Beta-wave frequencies from 14 to 30 Hz: This band of frequencies appears when sensory stimuli occur (i.e., in the normal waking state) or during mental tension.