The many nerve cells that make up the brain produce continuous electrical activity when a person is awake, asleep or even in a coma. This can be recorded using small metal discs electrodes, which are placed on the scalp. The electrical signals are then amplified by specialised equipment to produce EEG tracing as wavy lines, representing the fluctuations in electrical activity from moment to moment.
The EEG of someone sitting quietly with their eyes closed doing nothing in particular have a characteristic appearance. In this state, the alpha rhythm will commonly be recorded from the back of the head. This is one of the brain’s ‘resting rhythms’. It may become slowed or disappear altogether in many conditions affecting the brain, such as infections, coma or dementia. When the eyes are opened, the alpha rhythm may either disappear altogether or become less prominent.
During the EEG recording activation procedure such as hyperventilation may be carried out. This will commonly produce a change in the brain’s electrical activity and may bring out abnormalities not otherwise seen in the EEG. In children with absence seizures, hyperventilation may well provoke a fit. Observing the EEG during the fit will allow the diagnosis to be confirmed.
The second activation procedure used routinely is photic stimulation. Photic lights are known to produce a standard response in the brain, but in some people they provoke abnormal responses, including epileptic fits. People with photosensitive epilepsy may find that fits are brought on by things such as disco strobe lighting or sitting too close to a flickering TV screen. Photic stimulation during the EEG will help identify people who are photosensitive.