Ever wonder what information is contained in “brain waves,” (the EEG, electroencephalogram)? An EEG is a snapshot of brain electrical activity and is obtained by gluing small electrodes in a set pattern over the scalp in much the same way as EKG (electrocardiogram) electrodes are pasted on the chest. Most EEGs are recorded for only 30 minutes or so and therefore represent a small sample of what actually occurs during a 24 hour period.
Brain waves change with our level of alertness. They slow considerably during most stages of sleep and speed up during periods of extreme concentration. When we are awake but not involved in a mental task they idle within a fairly constant range. The EEG has characteristic patterns associated with different levels of sleep (for example during periods of dreaming). Because dream sleep is characterized by rapid eye movements this is called REM sleep. For this reason all night EEG recordings can be useful in diagnosing various sleep disorders.
Different drugs affect EEG patterns, so the test can be helpful in determining the cause of coma in patients. Complete loss of brain electrical activity is one factor in diagnosing brain death – the state in which the brain is irreversibly and totally damaged.
Some forms of epilepsy have specific patterns of activity associated with them. For this reason, the EEG is commonly used in diagnosing and treating seizure disorders. However, because these characteristic patterns come and go (as do the seizures) any EEG may miss their occurrence. Thus, although the presence of epileptic activity in an EEG is helpful in diagnosing the disorder, the absence of epileptic activity does not rule out the disorder.