Electroencephalogram (EEG)

Although Richard Caton recorded the electrical activity from the brain for the first time in 1875, Hans Berger was the first to record brainwaves from human beings in 1924. EEG recording is done by placing electrodes at designated places on the scalp or using cap fitted with electrodes (as shown in the figure below). Each electrode is connected to an input of a differential amplifier and a common electrode is connected to the other input of each amplifier. Thus, the voltage between the active electrode and the reference gets amplified.

	Sites of electrode placement

Four types of brain waves are recorded from the scalp of normal human beings using EEG— a lpha waves (8-13 Hz), beta waves (>13 Hz), theta waves (4-7 Hz), and delta waves (<4 Hz). Each of these waves appears as a single line representing the summated electrical field resulting from the activity of the neurons. These records can be unipolar or bipolar in nature. When an electrode is placed on the scalp and another on some other part of the body the obtained potential differences are called unipolar records whereas potential fluctuations between two cortical electrodes are termed as bipolar records.