ELECTROENCEPHALOGRAM: HISTORY, FUNCTION, WAVES - NEUROPSYCHOLOGY - 2025

The electroencephalogram (EEG) is a test used to record and evaluate the bioelectrical activity of the brain. The electrical potentials are obtained through electrodes located on the patient's scalp.

The records can be printed on moving paper via an EEG or can be viewed on a monitor. The electrical activity of the brain can be measured under basal conditions of rest, wakefulness, or sleep.

Application of electroencephalogram in child

The electroencephalogram is used for the diagnosis of epilepsy, sleep disorders, encephalopathies, coma, and brain death, among many other uses. It can also be used in research.

It was previously used to detect focal brain disorders such as tumors or stroke. Today, magnetic resonance imaging (MRI) and computed tomography (CT) are used.

Brief history of the electroencephalogram

The history of the electroencephalogram begins in 1870, when Fristsch and Hitzig, doctors in the Prussian army, investigated with the brains of soldiers. These were discovered in the Battle of Sedan. They soon realized that by stimulating some brain areas with galvanic current, movements were generated in the body.

Richard Birmick Caton

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However, it was in 1875 that physician Richard Birmick Caton confirmed that the brain produced electrical currents. This later allowed the neurologist Ferrier to experiment with the "faradic current", locating motor functions in the brain.

Vladimir Pravdich-Neminsky

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In 1913, Vladimir Pravdich-Neminsky was the first to perform what he called an "electrocerebrogram," examining the nervous system of a dog. Until that moment, all observations were made on discovered brains, as there were no enlargement procedures that reached the interior of the skull.

Hans berger

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In 1920, Hans Berger began experimenting with humans and 9 years later he created a method to measure the electrical activity of the brain. He coined the term "electroencephalogram" to characterize the recording of electrical fluctuations in the brain.

This German neurologist was the one who discovered the “Berger rhythm”. That is to say, the current "alpha waves", which consist of electromagnetic oscillations that come from the synchronous electrical activity of the thalamus.

Berger, despite his great discovery, I am unable to advance this method due to his limited technical knowledge.

In 1934, Adrian and Matthews, in a demonstration at the Society of Physiology (Cambridge) were able to verify the “Berger rhythm”. These authors advanced with better techniques and showed that the regular and wide rhythm of 10 points per second did not arise from the whole brain, but from the visual areas of association.

Frederic Golla

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Later, Frederic Golla confirmed that in certain diseases there were alterations in the rhythmic oscillations of brain activity. This allowed great advances in the study of epilepsy, becoming aware of the difficulty of this issue and the need to study the brain in a comprehensive way. Fisher and Lowenback, in 1934, were able to determine epileptiform peaks.

Finally, William Gray Walter, an American neurologist skilled in robotics, developed his own versions of the EEG and added improvements. Thanks to it, it is now possible to detect the various types of brain waves, from alpha waves to delta waves.

How does an electroencephalogram work?

A standard EEG is a painless, non-invasive scan performed by attaching electrodes to the scalp with a conductive gel. It has a recording channel, which measures the voltage difference between two electrodes. Typically 16 to 24 leads are used.

The pairs of electrodes are combined creating what is called a "mount", which can be bipolar (transverse and longitudinal) and monopolar (referential). The bipolar montage is used to record the voltage difference in areas of brain activity, while the monopolar compares an active brain zone and another with no or neutral activity.

The difference between an active zone and the average of all or some active electrodes can also be measured.

Invasive electrons (within the brain) can be used to study hard-to-reach areas such as the mesial surface of the temporal lobe in detail.

Electrocorticography

Sometimes it may be necessary to insert electrodes close to the surface of the brain, to detect electrical activity in the cerebral cortex. The electrodes are usually placed under the dura (one of the layers of the meninges) through an incision in the skull.

This procedure is called electrocorticography, and it is used to treat resistant epilepsy and for investigations.

10-20 system

There is a standardized system for electrode placement known as the "10-20 system." This implies that the distance between the electrodes should be 10% or 20% with respect to the frontal axes (from front to back) or transverse (from one side of the brain to the other).

21 electrodes must be placed, and each electrode will be connected to one input of a differential amplifier. The amplifiers spread the voltage between the active and the reference electrode between 1,000 and 100,000 times.

At present, the analog signal is in disuse and digital amplifiers are used. Digital EEG has great advantages. For example, it facilitates the analysis and storage of the signal. In addition, it allows modifying parameters such as filters, sensitivity, recording time and montages.

EEG signals can be recorded with open source hardware such as OpenBCI. On the other hand, the signal can be processed by free software such as EEGLAB or the Neurophysiological Biomarker Toolbox.

The electroencephalographic signal is represented from the difference in electrical potential (ddp) that exists between two points on the cranial surface. Each point is an electrode.

EEG brain waves

Our brain works through electrical impulses that travel through our neurons. These impulses can be rhythmic or not, and are known as brain waves. The rhythm consists of a regular wave, which has the same morphology and duration, and which maintains its own frequency.

Waves are classified according to their frequency, that is, according to the number of times the wave repeats per second, and they are expressed in hertz (Hz). The frequencies have a certain topographic distribution and reactivity. Most of the brain signal observed on the scalp is in the range between 1 and 30 Hz.

On the other hand, the amplitude is also measured. This is determined from the comparison of the distance between the baseline and the peak of the wave. Wave morphology can be sharp, pointed, in tip-wave complexes and / or sharp wave-slow wave.

In the EEG, 4 main bandwidths known as alpha, beta, theta and delta can be seen.

Beta waves

Beta waves. Source: Hugo Gamboa

They consist of broad waves, whose frequency is between 14 and 35 Hz. They appear when we are awake doing activities that require intense mental effort, such as taking an exam or studying.

Alpha waves

Beta waves. Source: Hugo Gamboa

They are of greater amplitude than the previous ones, and their frequency oscillates between 8 and 13 Hz. They arise when the person is relaxed, without making significant mental efforts. They also appear when we close our eyes, daydream, or carry out activities that we have highly automated.

Theta waves

Beta waves. Source: Hugo Gamboa

They have a greater amplitude but a lower frequency (between 4 and 8 Hz). They reflect a state of great relaxation, prior to the beginning of sleep. Specifically, it is linked to the early stages of sleep.

Delta waves

Delta waves. Source: Hugo Gamboa

These waves are the ones with the lowest frequency of all (between 1 and 3 Hz). They are associated with deeper stages of sleep (stages 3 and 4, where you don't usually dream).

Process

To perform the EEG, the patient needs to be relaxed, in a dark environment and with his eyes closed. It usually lasts about 30 minutes.

Initially, activation tests such as intermittent photostimulation (applying light stimuli with different frequencies) or hyperventilation (breathing through the mouth regularly and deeply for 3 minutes) are performed.

It can also induce sleep or, conversely, keep the patient awake. This depends on what the researcher intends to observe or verify. This video shows the application in an adult:

Interpretation

To interpret an electroencephalogram, it is necessary to know the normal activity of the brain according to the age and condition of the patient. It is also necessary to examine artifacts and possible technical problems to minimize misinterpretations.

An EEG may be abnormal if epileptiform activity is present (suggesting an epileptic process). This can be localized, generalized or with a particular and unusual pattern.

It can also be abnormal when slow waves are visualized in a specific area, or generalized asynchrony is found. There may also be abnormalities in amplitude or when there is a line that deviates from normal.

Currently, other more advanced techniques have been developed such as video-EEG monitoring, ambulatory EEG, telemetry, brain mapping, in addition to electrocorticography.

Types of electroencephalogram

There are different types of EEGs which are listed below:

Baseline electroencephalogram

It is the one that is performed when the patient is in a waking state, so no preparation is required. To avoid using products that may affect the examination, a good cleaning of the scalp is carried out.

Electroencephalogram in sleep deprivation period

Previous preparation is necessary. The patient must be awake for 24 hours before its performance. This is done in order to be able to make physiological traces of the sleep phases in order to detect abnormalities that cannot be obtained through the baseline EEG.

Video-electroencephalogram

It is a normal EEG, but its distinctive feature is that the patient is videotaped during the process. Its purpose is to obtain a visual and electrical record to observe if crisis or pseudo-crisis appear.

Electroencephalogram of brain death

It is a necessary technique to observe the cerebral cortical activity or its absence. It is the first step of the so-called “brain death protocol”. It is essential to start the device for the extraction and / or transplantation of organs.

Clinical applications

The electroencephalogram is used in a wide variety of clinical and neuropsychological conditions. Here are some of its uses:

Detect epilepsies

The EEG in epilepsies is essential for diagnosis, as it allows it to be differentiated from other pathologies such as psychogenic crises, syncope, movement disorders or migraines.

It is also used to classify epileptic syndrome, as well as to control its evolution and the effectiveness of treatment.

Detect encephalopathies

Encephalopathies involve damage or malfunction of the brain. Thanks to the electroencephalogram, it is possible to know if certain symptoms are due to an "organic" brain problem, or are the product of other psychiatric disorders.

Control anesthesia

The electroencephalogram is useful to control the depth of anesthesia, preventing the patient from entering a coma or waking up.

Monitor brain function

The EEG is essential in intensive care units to monitor brain function. Especially seizures, the effect of sedatives and anesthesia in patients in an induced coma, as well as to check for secondary brain damage. For example, the one that can occur in a subarachnoid hemorrhage.

Abnormal operation detection

It is used to diagnose abnormal changes in the body that can affect the brain. It is usually a necessary procedure to diagnose or monitor brain diseases such as Alzheimer's, head injuries, infections or tumors.

Certain electroencephalographic patterns may be of interest for the diagnosis of some pathologies. For example, herpetic encephalitis, cerebral anoxia, barbiturate poisoning, hepatic encephalopathy, or Creutzfeldt-Jakob disease.

Check for proper brain development

In newborns, the EEG can provide information about the brain to identify possible abnormalities based on their life span.

Identify coma or brain death

The electroencephalogram is necessary to assess the patient's state of consciousness. It provides data on both the prognosis and the degree of slowing of brain activity, so that a lower frequency would indicate a reduction in the level of consciousness.

It also allows us to observe whether brain activity is continuous or discontinuous, the presence of epileptiform activity (which indicates a worse prognosis) and reactivity to stimuli (which shows the depth of the coma).

In addition, through it, the presence of sleep patterns (which are infrequent when the coma is deeper) can be verified.

Pathologies in sleep

The EEG is very important for the diagnosis and treatment of multiple sleep pathologies. The patient can be examined while they sleep and their brain wave characteristics observed.

The most widely used test for soil studies is polysomnography. This, in addition to including an electroencephalogram, simultaneously records the patient on video. In addition, it allows to analyze your muscle activity, respiratory movements, air flow, oxygen saturation, etc.

Investigation

The electroencephalogram is used in research, especially in neuroscience, cognitive psychology, neurolinguistics, and psychophysiology. In fact, many of the things we know about our brain today are due to research done with EEGs.

References

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