With the EEG, the electrodes are attached directly to the scalp.

Electroencephalography, or EEG for short, is used to measure and display potential fluctuations in nerve cells in the cerebrum.

The basis for this is the change in the electrolyte concentration (electrolytes = salts) in the intra- and extracellular space when the cell is excited. It is important that the EEG does not record individual action potentials, but rather the total potential of larger units of nerve cells (neurons).


The electroencephalogram is extreme cheap and easy to do diagnostic method.

To measure the total potential, a certain number Electrodes with a gel on defined points of the Scalp appropriate. In addition, a reference electrode must be attached to a point on the head where there are few interfering signals. Often an area will be am ear elected. This has the advantage of being there little muscular tissuer, which in the event of an unwanted contraction leads to a falsification of the EEG signal. Generally speaking, the patient should be his Facial muscles relax and the Keep your gaze as straight as possible.

The electrical currents measurable by the scalp are extremely lowbecause there is a lot of poorly conductive tissue between the nerve cells of the cerebrum and the measuring electrode. Therefore, the signals must use a Amplifier can be made visible on a monitor. The magnitude of a deflection is in the range of one Microvolt.

A major disadvantage of the EEG is that poor spatial resolution of the procedure. This is because the activity of individual nerve cells is too weak to be registered. First the signal from big ones Groups of neurons (several nerve cells) is strong enough to be detected by the electrodes on the scalp. With electroencephalography, it is only possible to determine to the nearest centimeter in which brain region the measurement results are recorded. If you want to achieve the most precise localization possible, you use the so-called Electrocorticography. In this neurosurgical procedure, the measuring electrodes are attached directly to the surface of the cerebrum after the skull has been opened and the measurement is started. Since this way only very little interfering tissue between signal and receiver the activity of even very small groups of neurons can be displayed on the monitor. The main purpose of this method is to be able to measure the neuronal activity of specifically selected brain regions. Of course, this method is a major surgical procedure that also involves risks, which is why it will only be used for more specific questions.

After all the preparations have been made and the EEG has been recorded, the question now arises: What am I actually seeing? If there is little interference, a wave appear, which, however, looks quite irregular to the layman. This is mainly due to the fact that not only the potential fluctuations are measured on a single neuron (nerve cell), but by several thousand nerve cells, some of which work independently of each other. That is why the doctor is not interested in a regular curve shape with the EEG, he rather pays attention Frequency (number of oscillations per unit of time) and amplitude (maximum deflection) of the waves. The amplitude of an EEG wave depends largely on the Synchronicity of the nerve cells involved. This means that the more neurons are active at the same time and work synchronously, the higher the amplitude in the EEG. Many nerve cells work intensively, but independently of one another, so the amplitude is low while the frequency is very high. According to this principle, different types of EEG waves are distinguished, which play an important role in the evaluation of electroencephalography.


Depending on the question, different parameters are taken into account when evaluating the electroencephalogram. To characterize the EEG waves, their frequency certainly.

When the neurons of the cerebrum are stressed, such as when solving a difficult brain teaser, the EEG can generate waves with a frequency of 30-80 Hz (Hz = Hertz, unit of frequency, 1 Hz = 1 wave per second). These types of waves in electroencephalography are called gamma-Waves designated.

So-called beta-Waves have a frequency between 15-30 Hz and above all join eyes open when awake on. The relatively high frequency comes through Sensory impressions that are processed in the brain.

The wave types with the next lower frequency are the alpha-Waves. They are in the frequency range between 10-15 Hz and are from the electroencephalogram at awake state but with closed eyes registered. The example of the alpha waves clearly shows that sensory impressions such as the See, leads directly to a reduction in the frequency in the EEG.

Are the Patient's eyes closed and it is in one light sleepso kick theta-Waves on. They have a frequency of 5-10 Hz.

The lowest frequency is at Deep sleep with the so-called theta-waves reached. Here you can only 3-5 waves per second (3-5 Hz) are recorded.

Electroencephalography is also an important part in the characterization of Sleep stages. In addition to the wave types already mentioned, so-called wave types occur during sleep Sleep spindles on. These show up in the EEG as short high-frequency discharges with a relatively high amplitude. They come in the first place Sleep stage II in front. Also at this stage, so-called k complexes to be watched. A k-complex is a section in the EEG with a very high amplitude but a low frequency and is probably associated with a high degree of synchronicity in thalamic nerve cells.

A final characteristic picture in the EEG are spike-and-wave complexes. These high frequency, high amplitude waves can occur during a epileptic seizure can be measured in the electroencephalogram. The spike-and-wave complexes are due to a pathological (morbid) Overactivity specific nerve cells in individual brain regions during an attack.


With the help of electroencephalography (EEG) an electroencephalogram is created, on which the course and strength of the bioelectrical activity of the brain are recorded. This electroencephalogram contains waves that follow certain frequency patterns (Frequency bands), Amplitude patterns, local activity patterns and their frequency of occurrence can be evaluated. 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 processes (e.g. spectral analysis) can also be used for evaluation. The are particularly rich in information in the evaluation Frequency bandswhich can generally be divided into four categories:

Delta waves

Frequencies from 0.5 to 3 Hz: This frequency band can be observed particularly in deep sleep and is characterized by slow and large amplitudes in the electroencephalogram.

Theta waves

Frequency 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 the awake adult, the permanent occurrence of theta waves (and also delta waves) is to be assessed as a noticeable finding.

Alpha waves

Frequencies between 8 and 13 Hz: These frequencies represent the basic rhythm of the bioloelectric activity of the brain and appear in the electroencephalogram when the patient's eyes are closed and the patient is in a resting state.

Beta waves

Frequencies from 14 to 30 Hz: This frequency band shows itself when sensory stimuli occur (i.e. in the normal waking state) or when mental tension.

Electroencephalography and Sleep

It was only with the help of electroencephalography that researchers succeeded in making those known today Sleep stages define. Especially the different wave frequencies and other peculiarities like Sleep spindles or k complexes help distinguish.

A normal sleep cycle will first be described. If you close your eyes, you can see the EEG alpha-Waves can be displayed with low amplitude. During the Falling asleep these waves change. On the one hand, the frequency drops, one speaks of theta-Waves. In addition, an increase in the amplitude of individual waves can be observed. Basically, it can be said that the deeper you sleep, the frequency decreases continuously while the amplitude increases. This leaves a high synchronicity of the nerve cells of the cerebrum during sleep.

The Sleep stage I. is only a few minutes long and has a low wake-up thresholdThis means that only a weak external stimulus is needed to wake people up. This follows stage I sleep Sleep stage II. This is with about 15 minutes a little longer and also has a higher wake-up threshold. The electroencephalogram shows theta-Waves measurable with greater amplitude compared to stage I. There are also specific k complexes and sleep spindles which are characteristic of stage II sleep. On the Sleep stage III With long wave delta waves finally follows that Stage IV. This is characterized by delta-Waves with high amplitude. In addition, this sleep stage has the highest wake-up threshold and lasts between 20-40 minutes. Although the consciousness is largely isolated from sensory impressions during deep sleep, very intense stimuli can still reach the brain and lead to waking up. This fact is a great advantage, especially in dangerous situations, because people can react as quickly as possible. The sleep stages III and IV are also based on their characteristics in the electroencephalogram as "slow-wave- “or synchronized sleep.

During deep sleep the dominates Parasympathetic nervous system in the body. He stimulates digestion, slows down breathing and slows the heartbeat. This is useful because the body should recover during sleep and to provide energy for the waking state.

After stage IV sleep, the rest of the sleep stages are reversed again until there is a significant change in the EEG after stage I has been reached. It will Waves of wakefulness (Beta waves) and the amplitude decreases sharply, although the wake-up threshold remains very high. One speaks of desynchronized sleep. It is mainly based on reactions of the Sympathetic dominates. The blood flow to the brain increases sharply, heartbeat and breathing rate increase. The penis or clitoris can also be aroused. The skeletal muscles are slack, only the eye and respiratory muscles show a certain tone. Since it is often too in desynchronized sleep Eye twitching and eye movements it will also come as "Rapid Eye Movement (SEM) “- means sleep. In addition, it should be noted that people who came from the REM sleep wake up being able to remember dreams more often. That is why it is assumed that people mostly dream in REM sleep.

In the first sleep cycle, REM sleep lasts about 10 minutes, but it gets a little longer with each cycle. Normally the person goes through one night between 5 and 7 sleep cycles. Towards the end of sleep, REM sleep can be up to 40 minutes long. Often, sleep ends with this phase, although the wake-up threshold is comparatively high.

Clinical application

Some pathological changes in the brain can be visualized using the EEG. For example Circulatory disorders, attention disorders and sleep disorders can be diagnosed using this method.

A specific example is neurodegenerative disease multiple sclerosis. In its course, the insulating layer around the nerve cells breaks, so that its function as a mediator of sensory impressions is restricted. The nerve cells then transmit information more slowly and information is lost due to the lack of isolation. The EEG can be used to record the time between the arrival of a stimulus and the actual measurement (latency). The latency of such sensory evoked potentials is typically prolonged in multiple sclerosis.

Another classic application example of the EEG is the recording of epileptic seizures. One differentiates between one partial epilepsywhich only affects certain brain regions, and one generalized epilepsythat includes the whole brain. If there is a seizure, electroencephalography is carried out so-called "spike and wave complexes visible. These are characterized by high synchronicity, i.e. high amplitudes in the EEG.

Another important application example is the Diagnosis of brain death to call. They show up in a brain-dead patient no amplitudes on the electroencephalogram. In this case one speaks of a isoelectric or Zero-line EEG. This joins Inactivity of the cerebrum, cerebellum, and brain stem and is therefore a clear indication of brain death. Because the brain activity even with the most modern machines Not be restored and therefore counts as definitive sign of death.


Electroencephalography is a relative cheap and entertaining diagnostic procedure. The routine exam will take no longer than one half a hour and costs between 50 and 100 €. If there is a justified suspicion of an illness, the procedure will be covered by the health insurance company.