The electroencephalography ( EEG , from ancient Greek ἐγκέφαλος enképhalos , German , brain ' , γράφειν Graphein , German , write' ) is a method of medical diagnosis and the neurological research to measure the summed electrical activity of the brain by recording the voltage fluctuations at the head surface. The electroencephalogram (also abbreviated to EEG ) is the graphic representation of these fluctuations. In addition to electroneurography (ENG) and electromyography (EMG), the EEG is a standard examination method in neurology .
These fluctuations in potential are caused by physiological processes in individual brain cells that contribute to the brain's information processing through their electrical changes in state. According to their specific spatial arrangement, the potentials generated by individual neurons add up, so that potential changes distributed over the entire head can be measured.
For clinical evaluation, a record in at least twelve channels from different electrode combinations is required.
The spatial resolution of the usual EEG is several centimeters. If a higher spatial resolution is required, the electrodes must be placed directly on the cerebral cortex to be examined after neurosurgical opening of the skull . However, this is only in special cases such. B. required before epilepsy surgery. In this case one speaks of an electrocorticogram ( ECoG ; in German spelling electrocorticogram ). The ECoG enables a spatial resolution of less than 1 cm and also offers the possibility of testing the function of the cerebral cortex below by means of selective electrical stimulation of one of the electrodes. This can be done for the neurosurgeon e.g. This can, for example, be of the greatest importance in the case of interventions in the vicinity of the linguistic region in order to decide which parts to remove without having to fear a loss of function (cf. awake craniotomy ). An even more detailed recording of single cell activity is only possible in animal experiments.
The resulting data can be examined by trained specialists for unusual patterns. There are also extensive software packages for automatic signal analysis . A widely used method for analyzing the EEG is the Fourier transformation of the data from the time domain (i.e. the familiar representation of voltage changes over time) into the so-called frequency domain . The representation obtained in this way allows the rapid determination of rhythmic activity.
In 1875 Richard Caton in Liverpool showed that the brains of monkeys and rabbits generate weak electrical currents that arise in the cerebral cortex and that can be influenced by sensory stimuli. Since then, Caton has been considered the discoverer of electroencephalography, especially in the Anglo-American region.
The Russian Nikolai Prawdicz-Neminski is one of the researchers who confirmed Caton's discovery in animal experiments and coined the term electrocerebrogram in 1925 .
The term electroencephalography goes back to the neurologist Hans Berger , who carried out the first electroencephalographs of humans at the University of Jena in 1924 . He called them "electrenkephalograms". The work published in 1929 was also noticed by the popular scientific press. The magazine Kosmos reported in 1930:
"In people with gaps in the skull, he put needle electrodes on the gap and connected them to a highly sensitive galvanometer, whereby he received a regular curve."
Berger also discovered the phenomenon of the alpha block (also known as the Berger effect). This is a very noticeable change in the EEG that sets in when a healthy subject opens his eyes or is encouraged to increase mental activity.
Since the signals to be measured on the scalp are in the range of 5 to 100 µV (1 microvolt = 1 millionth of a volt), a sensitive measuring amplifier is required. A differential amplifier with high common-mode rejection is used to suppress the omnipresent mains hum and other interference . For reasons of patient safety , this is implemented as an isolation amplifier in electroencephalographs approved as medical devices, whereby the common-mode rejection is also increased at the same time.
The devices used before the use of computers conducted the output of the differential amplifiers to a measuring recorder , and the electroencephalogram was written on continuous paper . The amount of paper corresponded to about 120 sheets for a standard test of 20 minutes.
Electrical voltages are always measured between two points. The electrodes for the EEG are each attached in a specific system, according to which different types of leads are differentiated. The 10-20 system is common ; however, alternative assemblies such as the 10-10 system and invasive discharges are also used.
With paperless or computer EEG, the signal is digitized and recorded on hard drive or optical media and the EEG is usually evaluated on the screen by the neurologist or psychiatrist.
Related and derivative methods
Evoked potentials and event-related potentials are derived by averaging EEG sections that follow certain stimuli . This sometimes requires a greater bandwidth and sensitivity of the amplifiers, especially in the case of early acoustic evoked potentials .
Another method for measuring brain waves, which is also widely used in medical technology, is indexing the brain waves using their magnetic field, which is measured using SQUID technology (see also magnetoencephalography ).
EEG frequency bands and grapho elements
The macroscopically visible electrical brain activity can have motifs that resemble rhythmic activity. Basically, however, the EEG is similar to 1 / f noise and does not contain any long-lasting oscillations .
Different degrees of alertness are accompanied by changes in the frequency spectrum of the EEG signals, so that vague statements about the state of consciousness can be made by analyzing the measured voltage curves .
The EEG is often divided into frequency bands (so-called EEG bands), with the number of bands and the exact classification being given differently by different authors. The division of the frequency bands and their limits are historical and do not always coincide with limits that are considered reasonable on the basis of more modern studies. For example, the theta band was divided into a range of theta 1 and theta 2 in order to take account of the different meanings of the sub-areas. In neurofeedback , the 12 to 15 Hz range is also known as the SMR band (Sensorimotor Rhythm).
The EEG evaluation is traditionally done by pattern recognition by the trained evaluator. In particular for long-term and sleep EEGs, software algorithms are also used for assisted or automatic evaluation, which are intended to simulate this pattern recognition. This is easier for the EEG bands, which are mainly defined in the frequency range, and somewhat more difficult for other grapho elements , typical patterns in the EEG.
For example, For example, a very asynchronous pattern of all frequency bands indicates strong emotional stress or loss of voluntary control, while an increasing number of slow waves with simultaneously few fast waves indicate a state of sleep or dozing.
Delta waves have a low frequency of 0.1 to <4 Hz. They are typical of the mostly dreamless deep sleep phase (N3, slow wave sleep SWS). In infants they are physiological even in the waking EEG, in older children they can be embedded in the normal occipital basic rhythm ( delta de jeunesse , posterior slow waves of the youth ). Under pathological conditions, they also occur focally (circumscribed) or generalized (diffuse) when awake and then indicate a brain dysfunction or brain lesion. Examples are delta (theta) foci in circumscribed deep brain lesions (e.g. cerebral hemorrhage , cerebral infarction , brain tumor ), frontal intermittent delta activity (FIRDA) in the context of brain dysfunction and temporal, intermittent delta activity in temporal lobe epilepsies ( Temporal lobe epilepsies).
A signal in the frequency range between 4 and <8 Hz is called a theta wave. They occur more frequently with drowsiness and in the light sleep phases N1 and N2. In the waking state they are physiological in small children, in adults they can indicate a brain dysfunction or a brain lesion (see Delta waves).
A signal in the frequency range between 8 and 13 Hz is called an alpha wave. An increased proportion of alpha waves is associated with slight relaxation or relaxed wakefulness with closed eyes. Alpha waves are considered to be an emergent property. Alpha waves mainly occur when the eyes are closed and are replaced by beta waves when the eyes are opened ( Berger effect ). The same effect can be achieved with closed eyes, if you z. B. begins to solve a simple arithmetic problem in your head.
A signal in the frequency range between 13 and 30 Hz is called a beta wave. Beta waves have various causes and meanings, such as: B. beta waves occur in around 8% of all people as a normal EEG variant. Beta waves also arise as a result of the effects of certain psychotropic drugs or REM sleep . Physiologically, β oscillations also occur z. B. when constantly tensing a muscle or with active concentration.
A signal in the frequency range above 30 Hz is called a gamma wave. It occurs, for example, with strong concentration, learning processes or meditation. In monks with longstanding meditation practice, amplitudes that are more than 30 times higher are measured. Recent research has shown the appearance of the gamma band in the so-called top-down regulation and the synchronization of different brain areas to integrate different qualities of a stimulus. They cannot be seen with the naked eye on an EEG strip.
Steep waves ( English sharp waves ) denote, as their name suggests, steeply rising or falling EEG lines. They are typical epilepsy potentials (ETPs). They last about 80–200 ms, protrude from the basic activity and must be distinguished from the shorter spikes.
EEG patterns with sharp waves, e.g. B. for rolando epilepsy with centro-temporal sharp waves. In Creutzfeldt-Jakob disease , too, there is a noticeable EEG with periodic sharp-wave complexes and in Martin-Bell syndrome , focal sharp waves can be detected.
Sharp slow wave
A sharp slow wave is a sharp wave with a subsequent high-tension slow wave. These are mostly delta waves. Sharp slow waves can also occur in complexes. This is the case if the frequency is below 3 Hz.
A spike wave is a subsequent high-tension slow wave. These are mostly delta waves. If these occur over a longer period of time, one speaks of so-called spikes-waves complexes. SW complexes usually appear in groups or series, but mostly generalized. The “classic” SW complex has a frequency around 3 Hz and occurs more frequently in REM sleep phases.
Slow cortical potentials
Slow cortical potentials (SCP) , slowly changing cortical potential fluctuations, are potential fluctuations in the order of 100 to 200 µV and a duration of one to a few seconds. These are many times larger than the EEG waves delta to gamma, but are not visible in a conventional electroencephalogram, as they areusually filtered outusing a high-pass filter.
Sleep spindles are typical wave patterns for the non-REM sleep phase 2 (N2), but can also sporadically be detectable in stage 1 (N1) and also occur in deep sleep (N3, slow wave sleep SWS). They arise from feedback in thalamocortical networks and indicate an inhibition of perceptual stimuli in the thalamus . Sleep spindles are therefore considered to be sleep stabilizing. Two types of spindles can be distinguished by frequency analysis: Spindles with 11.5–13.5 Hz have a frontal maximum, spindles with 12.5–14.5 Hz have a central maximum. Under barbiturates (. Eg phenobarbital and) benzodiazepines (eg. Diazepam ), there is an increase in frequency and acceleration (up 15.5Hz) of spindles.
K-complexes are wave patterns that typically occur in the non-REM sleep phase N2, can already be detected in isolated cases in the wake-to-sleep transition (N1) and continue in deep sleep (N3, SWS). The waveform is biphasic (see fig.) With a steep upward stroke to the negative maximum, a somewhat slower decline to positivity and then a return to the zero line. The amplitude is over 75 µV, sometimes over 200 µV. A sleep spindle can follow immediately. Arousal reactions ( arousals ) can also be initiated by K complexes. Physiologically, it is an EEG activation by external or internal stimuli, so that K complexes can also be viewed as a form of evoked potentials.
Vertex waves are characteristic of the wake-sleep transition, but also occur in the further course of sleep, especially in stable light sleep. They have a duration of less than 200 ms, are largely symmetrical, and show a sharp negative peak. Its maximum is above the vertex . Physiologically, it is probably a stage-specific subgroup of the K complexes.
Applications in medicine
The electroencephalogram is a standard neurological examination .
The electroencephalogram is used to diagnose and monitor the course of epilepsy . In addition to the high-amplitude activity during a seizure, specially shaped grapho elements are also noticeable in the seizure-free interval.
The extinction of the “brain waves” (i.e. the absence of voltage fluctuations in the EEG) is an auxiliary criterion in determining brain death .
Depth of coma and anesthesia
Using specific criteria, which relate to grapho elements and frequency modulation of the EEG, the depth of coma and anesthesia can be determined.
In sleep medicine , an all-night EEG is recorded (often with a reduced set of electrodes). From this information can be obtained about the latency to sleep, the distribution of the sleep stages (shown as a hypnogram ), waking reactions (spontaneous or as a result of external or internal sources of interference such as noise or sleep-related breathing disorders) and other physiological and pathological processes during sleep. Usually the EEG is combined with the measurement of other physiological parameters in the context of polysomnography . With the EEG, which is used for sleep stage analysis in the sleep laboratory, only a few recordings are usually made - compared to the complete EEG of the 10-20 system. By default, according to Allan Rechtschaffen and Anthony Kales , the channels C3 / A2, C4 / A1 are derived.
Applications outside of medical diagnostics
Specific functional phases of the brain can be assigned to the above-mentioned basic frequencies in the brain waves. With the aid of computer-aided frequency analyzes such as FFT , the transitions can also be analyzed in real time .
The column 'Prioritization' refers to the priority of the brain performance observed in this phase, which can possibly be achieved through targeted stimulation of the brain activity or also spontaneously e.g. B. by overstimulation or stimulus deprivation ( meditation ). The form of this orientation is still in the scientific discussion.
|Delta (δ)||0.5- <4 Hz||Deep sleep , trance|
|Theta (θ)||Low (theta 1)||4-6.5 Hz||Hypnagogic awareness (falling asleep), hypnosis , waking dreams|
|High (theta 2)||6.5- <8 Hz||Deep relaxation , meditation , hypnosis , waking dreams||Increased ability to remember and learn, concentration , creativity , relief of the meditation state|
|Alpha (α)||8-13 Hz||Light relaxation , super learning (subconscious learning ), inward-looking attention, closed eyes||Increased ability to remember and learn|
|Beta (β)||Low (Sensorimotor Rhythm, SMR)||> 13-15 Hz||Relaxed outward-facing attention||Good receptivity and attention|
|medium||15-21 Hz||Wide awake, normal to increased outward alertness and concentration||Good intelligence|
|High||21-38 Hz||Hustle and bustle , stress , fear or overactivation||Erratic thought leadership|
|Gamma (γ)||38-70 Hz||Demanding activities with a high flow of information||Transformation or neural reorganization|
Determination of the intelligence quotient
Many scientific publications suggest that EEG measurements can be used to estimate the intelligence quotient (IQ). In particular, the power density of the alpha and beta bands correlates with the IQ. To what extent the method can be used to determine in individual cases apart from statistical values is controversial.
Influencing the brain waves
Brain waves can not only be measured, but also influenced. This can be done by a visual or acoustic stimulus, by neurofeedback or by direct manipulation of the brain waves using alternating electrical fields (see also: Transcranial Magnetic Stimulation , TMS). Devices that are supposed to make this possible have been available under the name Mindmachines or Brainwave Stimulator since the 1980s , with controversial success.
Neurofeedback is based on the above. Classification of the EEG tapes and related functional phases are referred to. In addition, an increased amplitude within a frequency range is sometimes correlated with specific mental states or activities.
The scientifically controversial hemispheric synchronization is a method that tries to change brain activity in such a way that brain waves of the same type are measured in both hemispheres.
Control by brain waves
Recent research under the heading of Brain-Computer-Interface (BCI) is making progress in direct control of computers through cognitive processes. Among other things, test subjects from the New York State Department of Health, the State University of New York in Albany and the Graz University of Technology (Laboratory of Brain-Computer Interfaces) can use the EEG to precisely move a mouse cursor after some practice. This is a huge step forward when you consider that previous studies with animals and humans still worked with implanted wires to measure brain waves. These were treated as foreign bodies and rejected by the body; the monkeys in question only survived a few months.
In the meantime, brain-computer interfaces have already found their way into medical practice using the EEG and are used by severely paralyzed people to communicate with the outside world. In addition, they can also express their creativity using brainpainting .
The extent to which control via EEG is used in military technology is not unrestrictedly accessible to the public. What is certain is that for years there have been test projects for the short-term "disembodied" control of fighter jets under extreme acceleration loads. However, the trend here is more towards a purely machine control, since the high G loads also affect the reliability of human consciousness.
In an experiment in 2014, researchers succeeded in converting simple thoughts into binary signals via EEG . These were then transmitted from India to France via the Internet. Using transcranial magnetic stimulation , the signal was transmitted to the participant's brain, which enabled them to perceive flashes of light at the edge of their field of vision and thereby understand the message received (in the form of binary zeros and ones). With the experiment, the researchers wanted to find out whether it is possible to enable direct communication between two people by reading out and injecting brain activity.
Examples in science fiction, fantastic literature and art
In the thriller Firefox by Craig Thomas , an experimental is jet fighter controlled by thoughts. A helmet converts the signals from the brain into control commands. The idea of controlling devices through the power of thought also appears in many science fiction books.
A current example from the film Matrix is also the idea of connecting the brain directly to a computer and thus interacting with a virtual world. This idea originally came from William Gibson ( Neuromancer ).
In the film Futureworld - The Land of Tomorrow , dreams can be recorded on video by converting the sleeper's brain waves into image signals.
In the light novel series Sword Art Online , a device called NerveGear is used, which can read out and stimulate both the sensory and motor areas in the brain in order to create an artificial computer game reality.
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