EEG, or Electroencephalograph, is the recording of electrical activity along the scalp. It is a tool to study the brain while it is performing a cognitive task. This allows to detect the location and magnitude of brain activity involved in the various types of cognitive functions.
EEG is non-invasive and does not involve any X-rays, radiation, or injections. EEG has been used for many years and are considered very safe. The electrodes record activity without producing any sensation. Slight redness may occur in the locations where the electrodes were placed, but this will wear off after a few hours.
An EEG machine is a recording device connected by wires to electrodes pasted at key points on the patient’s head. The electrodes pick up signals produced by electrical discharge of neurons in the related areas of the brain; the amplified signal from each electrode causes pens writing on a moving belt of paper to jump—similar to the action of a seismograph when an earthquake occurs.
The resulting EEG tracing, with its record of electrical discharge, provides a record of activity in key areas of the brain during the period of the test. Excessive discharge (of the type that, if large enough, might cause a seizure) may show up as a sharp spike or series of spikes; some patterns (such as the 3-per-second spike and wave of absence seizures) are unique to particular forms of epilepsy.
EEG recordings of patients while awake are made with the eyes open and with the eyes closed. A flashing light is used to assess whether the patient is photosensitive—that is, if he or she will have a seizure in response to the stimulus of a flashing light.
If standard recordings do not produce evidence of seizures, 24-hour EEGs, or portable home EEG monitoring devices may be used. Nasopharyngeal and sphenoidal electrodes (long wires inserted through the nose or inserted into the jaw muscle) may produce information unobtainable from regular recordings. Grid or depth electrodes may be implanted in the brain in a surgical procedure when patients are being evaluated for epilepsy surgery and it is vital to get precise information on where the seizure site is located.
If the type and cause of the seizures are unclear, a type of evaluation known as intensive monitoring may be undertaken. In this procedure, people are videotaped during an EEG recording session. The combined image of EEG tracings and visible behavior helps the physician diagnose the epilepsy and identify affected areas of the brain. Intensive closed circuit TV and EEG monitoring of this type also helps distinguish between true epileptic seizures caused by electrical discharge and non-epileptic seizures caused by psychological factors.