The term electroencephalography is a composition of the Greek expressions encephalon (brain) and graphein (writing). It is used to measure changes in the potential of the cerebral cortex using scalp-mounted electrodes.
All neurons in the brain have a so-called resting membrane potential, which changes when aroused. The change of state of a single nerve cell is not detectable from the outside; However, if larger neuronal assemblies are excited synchronously, 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 strengthened. In addition, 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 as well as healthy, research-relevant brain activities. Electroencephalography was developed in the 1920s by the neurologist and psychiatrist Hans Berger from Jena (1873-1941).
In healthy people, depending on the state of alertness and cognitive performance, electroencephalography finds characteristic rhythmic patterns of activity: in the wakeful, relaxed state with eyes closed, alpha waves (8-12 Hz) appear, with open eyes beta waves (13-30 Hz), In the case of mental exertion, gamma waves in the frequency range above 30 Hz appear.
In sleep, however, theta waves (4-8 Hz) and delta waves (<4 Hz) are typical. Fundamental deviations from these oscillations indicate neurological disease processes. Particularly important is the electroencephalography for the diagnosis and follow-up of epilepsy, in which seizure-like discharges of large nerve cell associations occur. Here the EEG helps to determine the type and duration of the seizures and (in case of focal epilepsy) to identify sources of seizure.
But electroencephalography is also used in other disorders of consciousness: in sleep medicine, a whole-night EEG is often derived. From the recorded hypnogram the sleep latency, duration and distribution of the sleep stages as well as wake-up reactions can be read. In most cases, the electroencephalography is combined with other physiological measurement methods such as polysomnography, z. B. with electrocardiography (ECG) or pulse oximetry (non-invasive determination of arterial oxygen content).
Thus, different sleep disorders such as insomnia, parasomnias or dyssomnia can be detected and objectified. In addition, electroencephalography helps to determine the depth of anesthesia, but also the depth of a coma. Electroencephalography is a tool for detecting brain death. Since the cerebral cortex constantly has electrical activity even when at rest, the absence of it 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 subtle and not directly readable, but must be filtered out using statistical software. Often, electroencephalography is used to measure reactions and reaction times to specific stimuli in experiments. For this purpose, electroencephalography is particularly suitable 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). The spatial resolution of electroencephalography, however, is relatively crude. In addition, only the electrical activity of the cerebral cortex is detected; Deep brain areas can only be examined indirectly (via their influence on the cerebral cortex) by means of electroencephalography.
Commercial and therapeutic application finds electroencephalography for several years in so-called brain-computer interfaces (BCI). This technique allows direct control of computers using brainwaves and is used for play purposes, but also allows severely paralyzed people to communicate with the outside world.
Electroencephalography is a completely safe and harmless method of examination. Only electrodes are glued to the outer scalp and derived anyway existing electrical signals. The patient or test person is thereby exposed to no radiation exposure or other danger. A routine examination takes about 20-30 minutes; with special questions a long-term electroencephalography may be necessary.