It is the gross differences between action potentials and resting potentials that allow the Electroencephalograph to measure electrical activity in the brain. The Electroencephalograph is able to do this by amplifying the small electrical fields that are found on the scalp with the aid of small electrodes. The Electroencephalograph is aided by the amplification of the signals on the scalp that have to pass through three layers of meninges, cerebral spinal fluid and a thick layer of skull.

The Electroencephalograph machine measures electrical activity in the brain in two ways. Firstly it measures the amplitude of the signals. Secondly the Electroencephalograph records frequency of signals emanating from the brain. The Electroencephalograph records the frequency in Hz, which are cycles per second. The frequencies are found using the Electroencephalograph have been classified into frequency bands. The first band, which is the delta frequency, is activity that is below 4Hz. Theta is 4 – 7 Hz, Alpha is 8 – 13 Hz and Beta is all frequencies above 13 Hz, (Binnie 1981).

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In his 1929 paper, Berger described a low amplitude rhythmic waveform with a 10 Hertz frequency in the occipital areas of the brain when his participant was resting with eyes closed. Berger named the activity the alpha rhythm. Clinically, the alpha rhythm is considered to be the most important aspect of an Electroencephalograph recording, (Binnie 1982). The alpha rhythm is responsive to changes in visual attention. The alpha rhythm is usually found in a normal subject who has his/her eyes closed. Opening of the eyes and concentrating on a cognitively demanding task will cause alpha to disappear. This is known as alpha blocking. A person can produce alpha in the awake state by staring into space or daydreaming. Alpha varies considerably between adults and children and also between individuals. As a diagnostic tool, the Electroencephalograph looks for symmetry and balance. Differences in those factors could be predictive of disorder or disease.

An example would be that of finding a 50% difference in amplitude between the cerebral hemispheres. This difference is found in less than 6% of the population and would therefore be viewed with suspicion by the Electroencephalograph technician. Furthermore, in a normal Electroencephalograph, right-handed people tend to present slightly higher amplitude in the right hemisphere. Therefore higher left hemispheric amplitude in a right handed person or asymmetry greater than 50% would raise suspicions of the Electroencephalograph technician who would investigate further.

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