EEG is the abbreviated form of electroencephalography which is a bio-medical process of recording the minute electric currents produced by the brains of human beings and other animals. The instrument for detecting and recording the electric current produced by the activity of the brain is called electroencephalograph. This was discovered by Hans Berger of Jena (Germany) in 1929 and found to have important clinical significance in the diagnosis of brain diseases.
The brain constantly generates minute electrical currents even during sleep and in deep coma. These can be recorded from the skull’s surface by means of small wires called electrodes attached to the scalp. In normal persons they have an electrical potential of about 100 microvolts. So to make such recordings the doctors use a machine that has about 20 equally-spaced electrodes which are attached to the scalp’s surface in accordance with the standard positions adopted by the International Federation of EEG. Electrode positions are carefully measured so that subsequent recordings from the same person can be compared with the earlier ones. The electrodes are connected to an amplifier which amplifies the voltages to 1,000,000 times. The current then moves an electromagnetic pen that makes a graph on a chart paper.
In the normal adult person most of the EEG recordings are made up of rhythmic oscillating waves regularly repeated at about 10 hertz (hertz is the unit of frequency). They are called alpha waves. They are best obtained from the back of the central nervous system or brain when the patient is in a relaxed position with the eyes closed. The waves disappear or are blocked when the eyes are opened.
EEG normally reveals more rapid rhythmical movement. It consists of smaller waves called beta waves that are repeated at every 18-25 hertz. These are related to the function of sensory motor parts of the brain. During sleep, the central nervous system cells generate electrical waves of even higher voltage but they are slowed down to 2 or 3 hertz. During coma very feeble waves are also recorded.
The irregular slow waves, one to three per second that arise from the localized area of the damaged region of the brain are called delta waves. Rhythmic slow waves at frequencies between 4 and 7 hertz are called theta waves. Slow waves suggest some abnormality in case of adults but not so in infants or young children. EEG patterns change gradually in the growing child until he has attained the age of 8 to 12 years. That is when the adult patterns appear. Every person has a different EEG pattern. Identical twins have almost identical patterns.
EEG has proved to be very efficient for studying the working of the brain and detecting many brain diseases. A doctor can know if a person’s brain is working properly or not by looking at the pattern of the graph as EEG provides indications about any abnormalities of the brain. Epilepsy and abnormal metabolic conditions can be detected through it. ‘Coma’ conditions can be studied from EEGs.