Electroencephalography, or EEG for short, is used to measure and display potential fluctuations of nerve cells in the cerebrum. The basis for this is the change in electrolyte concentration (electrolytes = salts) of the intra- and extracellular space during excitation of the cell. It is important that the EEG does not record individual action potentials, but rather the sum potential of larger units of nerve cells (neurons).
Functionality
The electroencephalogram is an extremely inexpensive and easy to perform diagnostic method. To measure the sum potential, a certain number of electrodes with a gel is applied to defined areas of the scalp. In addition, a reference electrode must be placed at a location on the head where there are few interfering signals.
Often an area on the ear is chosen. This has the advantage that there is little muscle there, which leads to a distortion of the EEG signal in case of unwanted contraction. In general, the patient should relax his facial muscles and keep his gaze as straight as possible.
The electrical currents that can be measured by the scalp are extremely low because there is a lot of poorly conducting tissue between the nerve cells of the cerebrum and the measuring electrode. For this reason, the signals must be made visible on a monitor with the help of an amplifier. The magnitude of a deflection is in the range of one microvolt.
A major disadvantage of the EEG is the poor spatial resolution of the procedure. This is because the activity of individual nerve cells is too weak to be registered. Only the signal from large neuron groups (several nerve cells) is strong enough to be recorded by the electrodes on the scalp.
Therefore, electroencephalography can only determine with centimetre precision in which brain region the measurement results are recorded. If one wants to achieve the most precise localization possible, one uses the so-called electrocorticography. In this neurosurgical procedure, after the skullcap has been opened, the measuring electrodes are placed directly on the surface of the cerebrum and measurement begins.
Since there is very little interfering tissue between the signal and the receiver, the activity of even very small groups of neurons can be displayed on the monitor. This method is primarily used 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 preparations have been made and the EEG is recorded, the question arises: What do I actually see? If there are few interfering signals, a wave should appear on the monitor, but to the layman it looks quite irregular. This is mainly due to the fact that the potential fluctuations are not only measured at a single neuron (nerve cell), but at several thousand nerve cells, which partly work independently of each other.
For this reason, the physician is not interested in a regular course of the EEG curve, but rather in the frequency (number of oscillations per time unit) and amplitude (maximum deflection) of the waves. The amplitude of an EEG wave depends largely on the synchronicity of the nerve cells involved. That means, the more neurons are active and synchronously working at the same time, the higher the amplitude in the EEG. If many neurons are working intensively but independently of each other, 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.
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