Electroencephalography (EEG) is a noninvasive procedure for measuring electrical brain activity. In German, it is also referred to as brain wave measurement. Electroencephalography is completely harmless and is routinely used in medical diagnostics as well as for research purposes.
What is electroencephalography?
Electroencephalography is the measurement of potential fluctuations in the cerebral cortex using electrodes placed on the scalp. The term electroencephalography is a composition of the Greek terms encephalon (brain) and graphein (to write). It refers to the measurement of potential fluctuations of the cerebral cortex with the help of electrodes attached to the scalp. All neurons in the brain have what is called a resting membrane potential, which changes when excited. The change in state of a single neuron cannot be detected from the outside; however, if larger neuron clusters are synchronously excited, the potential changes add up and can also be measured outside the skull. Since the signal is attenuated by skull bones, meninges, etc. and is only in the μV range, it must be additionally amplified. Furthermore, interfering noise must be filtered out. The measured potential fluctuations are graphically displayed over time in an electroencephalogram. From these EEG curves, trained experts can read out disease processes, but also healthy brain activities relevant to research. Electroencephalography was developed in the 1920s by Jena neurologist and psychiatrist Hans Berger (1873-1941).
Function, effect, and goals
In healthy humans, electroencephalography finds characteristic rhythmic activity patterns, depending on the state of wakefulness and cognitive performance: in the awake, relaxed state with eyes closed, alpha waves (8-12 Hz) occur; with eyes open, beta waves (13-30 Hz) occur. During mental exertion, gamma waves appear in the frequency range above 30 Hz. During sleep, on the other hand, theta waves (4-8 Hz) and delta waves (<4 Hz) are typical. Fundamental deviations from these oscillations indicate neurological disease processes. Electroencephalography is particularly important for the diagnosis and follow-up of epilepsies, in which seizure-like discharges of large clusters of nerve cells occur. Here, the EEG helps to determine the type and duration of the seizures and (in the case of focal epilepsy) to identify seizure foci. Electroencephalography is also used for other disorders of consciousness: In sleep medicine, an all-night EEG is often recorded. From the recorded hypnogram, among other things, the latency to fall asleep, duration and distribution of sleep stages, and wake-up reactions can be read. In most cases, electroencephalography is combined with other physiological measurement methods such as polysomnography, e.g. electrocardiography (ECG) or pulse oximetry (non-invasive determination of arterial oxygen content). In this way, different sleep disorders such as insomnias, parasomnias or dyssomnias can be detected and objectified. In addition, electroencephalography helps to determine the depth of anesthesia, as well as the depth of coma. Electroencephalography is a tool for determining brain death. Since the cerebral cortex shows constant electrical activity even in the resting state, an absence of the same is considered an indication of irreversibly dead tissue. In addition to its clinical applications, electroencephalography is also frequently used in research. Here, the relevant changes in the EEG curve are usually more subtle and cannot be read directly, but must be filtered out using statistical software. Electroencephalography is often used to measure reactions and reaction times to certain stimuli in experiments. Electroencephalography is particularly suitable for this purpose because it has a high temporal resolution (in the millimeter range). In this aspect, it is clearly superior to other examination methods, such as magnetic resonance imaging (MRI), computed tomography (CT) and positron emission tomography (PET). In contrast, the spatial resolving power of electroencephalography is relatively coarse. In addition, only the electrical activity of the cerebral cortex is recorded; deeper-lying brain areas can only be examined indirectly (via their influence on the cerebral cortex) using electroencephalography. Electroencephalography has been used commercially and therapeutically for several years in so-called brain-computer interfaces (BCI).This technology allows computers to be controlled directly using brainwaves and is used for gaming purposes, but also allows the severely paralyzed to communicate with the outside world.
Side effects and dangers
Electroencephalography is a completely safe and harmless examination method. Only electrodes are attached to the external scalp and electrical signals that are present anyway are derived. The patient or subject is not exposed to radiation or any other danger. A routine examination takes about 20-30 minutes; long-term electroencephalography may be necessary for special questions.
