In the BRAIN there is a regular, rhythmical change of electric potential, due to the rhythmic discharge of energy by nerve cells. These changes can be recorded graphically as electroencephalograms. They are useful in DIAGNOSIS: for example, the abnormal electroencephalogram occurring in EPILEPSY can be used to determine the type of epilepsy, its site of origin in the brain and possibly which treatment is likely to be most effective.

The normal waves, known as alpha waves, occur with a frequency of 10 per second. Abnormal waves, with a frequency of 7 or fewer per second, are known as delta waves and occur in the region of cerebral tumours and in the brains of epileptics. An electroencephalogram can assess whether an individual is awake, alert or asleep. It may also be used during surgery to monitor the depth of unconsciousness in anaesthetised patients.