What is Electro-encephalography?

In 1929, the German psychiatrist, Hans Berger, published his groundbreaking paper "Über das Elektrenkephalogramm des Menschen" [1]. Therein, he describes a remarkable electric effect which can be detected in a human subject by electrodes applied to the head. This effect consists of a rhythmic oscillation of electric potential, with a frequency of around 10 Hz, and appears when the subject lies quietly with eyes closed. When the subject opens their eyes, this effect vanishes completely.
Background: Modified Siemens-Verstärker- Elektrokardiograph (first EEG device)

Recording EEG in the clinic

HD-EEG: High density EEG recording with 256 channels.
Today, the electro-encephalogram is an established clinical test to detect electrical brain activity using small conductive discs, also known as electrodes, that are simply placed onto the scalp.

As high density EEG, shown on the left, with up to 256 electrodes is not yet the established standard, most clinics record only the conventional, low-density EEG. In this setting only between 19 and 26 electrodes are used.

In the clinic, EEG is the gold standard for diagnosing epilepsy, but many other brain disorder can be diagnosed, monitored and their progression predicted based on EEG.
Conventional EEG recording with 21 electrodes, marked in blue. The yellow sites are only used in HD-EEG. Today, low-density EEG still remains the clinical standard.

What does an EEG recording look like?

Brain cells, also known as neurons, communicate via electrical impulses. This communication never ceases as long as a human is clinically considered to be alive. In fact, EEG is conventially used to confirm brain death in a persistent coma, where electrical activity is consistently at a bare minimum.
Figure A on the left shows an EEG recording of a healthy awake person in eyes open condition. The electric activity shows up as wavy lines on the EEG recording.
Figure B shows the EEG of a person during a phase of deep sleep. Extended deep sleep phases are extremely important for maintaining overall brain health as the brain switches into "repair mode" during those phases with a significantly increased immune response.
Both recordings show distinct characteristics linked to the current cognitive state. These characteristics generalize over different individuals and populations, often even to brains of other mammals.
Figure A: EEG recording of a healthy, awake person in eyes open condition.
Figure B: EEG recording of a person during a phase of deep sleep.

Past, Present and Future: The Potential of EEG

For over 70 years, EEG recording devices are part of the basic equipment of any neurological clinic. The fact that EEG, a methodology conceived almost a hundred years ago, could not be replaced by a newer and more sophisticated modality, shows that EEG provides information inaccessible to devices such as MRI- or PET-scans, X-ray or SPECT.

The future of EEG, especially in combination with powerful machine learning methods, holds tremendous potential: Pre-symptomatic detection of neuro-degeneration or prediction of cognitive ability over the course of many years is possible today, based on EEG. And with EEG devices costing only a fraction of an MRI device, population-wide screenings, even by general practitioners, become affordable and might well be part of the future of EEG.
Image
EEG of Klaus Berger, age 15, son of Hans Berger, recorded in 1927. The upper trace shows the EEG, the lower trace indicates the time, where each oscillation lasts exactly 0.1 seconds.

References

  1. Über das Elektrenkephalogramm des Menschen  [link]
    Berger, H. (1929). Archives for Psychiatry and Nervous Diseases , 87 (1), 527-570.
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