Electroencephalogram (EEG) 1924 – Monitoring the Brain's Electrical Activity

Electroencephalography (EEG) is used to understand the activity of a large group of neurons rather than focusing on the activity of a single neuron (nerve cell). EEG is a measurement that provides a view of the generalised activity of the cerebral cortex, the outer layer of brain tissue, also called the brain's cortex.

In 1875, British physiologist Richard Caton placed monitoring electrodes directly on the brain surface and used a galvanometer (an electric current detector) to measure electrical activity in the brains of rabbits and monkeys. In 1924, German psychiatrist and neurologist Hans Berger, motivated by his interest in psychic phenomena and telepathy, recorded the first human EEG using sensitive galvanometers. He also discovered for the first time that EEGs differed between awake and sleeping people. 

Berger named a particular rhythmic EEG pattern corresponding to a relaxed state when patients closed their eyes "alpha waves". The first clinical department where EEG services were performed and charged for opened in 1937 at Massachusetts General Hospital.

Today, EEG

• continues to be used for purposes including the study of sleep patterns and various research applications. 
• It is also used as an indicator of brain death. 
• It helps diagnose tumours or epilepsy, a neurological disorder characterised by seizures. 
• EEGs can also be used to examine brain signals triggered by a stimulus (such as a sound or visual pattern). 

During recording, electrodes are attached to standard locations on the scalp, and many readings are taken simultaneously. Because nerve signals must pass through several layers of tissue and the skull, the electrical activity associated with a single nerve cell is not read; EEG corresponds to the collective activity of many underlying neurons, and this activity is amplified (increased) by electrical circuits before being recorded. To measure changes in blood flow associated with neural activity, magnetic resonance imaging (fMRI) can be used to provide more information about location. With electrocorticography, electrodes are placed directly on the brain to detect higher-frequency, lower-voltage components.

Historical figures mentioned in the article: Richard Caton (1842–1926), Hans Berger (1873–1941)

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