HOW IS THE ELECTRICAL ACTIVITY OF THE BRAIN MEASURED

HOW IS THE ELECTRICAL ACTIVITY OF THE BRAIN MEASURED?

Introduction to the Electroencephalogram (EEG)

S leep is comprised of rhythmically recurring sleep stages, which are defined by looking at brain waves, or an electroencephalogram (EEG), in conjunction with the electrooculogram (EOG), which records eye movements and the electromyogram (EMG), a measure of muscle activity. 

The discovery of brain waves Before the sleep stages are described, it is useful to look at how the brain waves were discovered, and what they are like. It was discovered in the 1820s that electrical current could cause the needle of a magnetic compass to fluctuate, and that this effect could be multiplied by the use of coils of wire. An instrument based on this observation was known as a galvanometer, named after Luigi Galvani (1737–98), an Italian physician and biologist who in 1791 observed that electrical currents could cause the limbs of a dead frog to twitch. 

This was one of the first observations that electrical currents might be involved in biological processes. Some years later, Richard Caton (1842–1926), a Scottish physiologist, used a galvanometer to detect electrical current from the brains of dogs and apes. In 1875 he reported that the current differed at various times, increasing in strength during sleep and preceding death, then disappearing upon death. 

A half century later, Hans Berger (1873–1941), a German psychiatrist, made the next great advance. Berger had been a mathematics student who dropped out and enlisted in the cavalry. One day, he was thrown from his horse, landing in the path of an oncoming cannon carriage, which barely managed to stop at the last moment. 

At the same time that Berger had this life-threatening experience, his sister living some distance away is said to have had a sudden sensation that he was in danger, and urged her father to contact him. Berger was so struck by this apparent “psychic energy” alarming his sister, that he went on to devote his life to exploring the brain and how objective measures of its activity might relate to subjective psychic processes.

 In 1929, he described recordings of the electrical waves in humans that had been reported by Caton in animals, and developed a way to record them on moving strips of paper. He coined the term “electroencephalogram” to describe his new discovery. He showed that the waves differed in waking, in sleep, and in anesthesia, and that sharp spiking patterns appeared during epileptic seizures. 

He described the alpha rhythm in a subject resting with closed eyes, and its disappearance and replacement by faster beta waves when the eyes were opened. These waveforms over the years have been divided into several wavebands according to their frequency (number of waves per second). 

Other information used in describing them involves their shape, amplitude (a measure of their energy), and location on the head (see also “Features of an oscillating system,” page 75). Subsequent to Berger’s work focusing on alpha and beta, a series of EEG bands have been further defined and are generally recognized to this day.