An electroencephalogram (EEG) is a test that records electrical activity in the brain. Brain cells create tiny electrical impulses for communicating with each other. The EEG picks up these impulses through tiny wires (electrodes) placed on your scalp. The impulses are amplified and digitally recorded by a computer. The recordings look like wavy lines (sometimes called brain waves). An EEG may be done when you are awake, asleep, or both.
An EEG is usually done to see if a person is having seizures, and if so, what type of seizures they are. The EEG can also look for changes in brain activity caused by head injury, tumor, infection, or other problems that affect the brain. In addition, an EEG may be used to evaluate brain activity in someone who is unconscious or in a coma.
Prior to having the EEG done, you must avoid having caffeine and taking medicines that affect the nervous system. Once the electrodes are attached and the computer is recording, you may be asked to do things like open or close your eyes, or change your breathing to fast or slow. You might be exposed to bright or flashing lights or noise. The EEG usually takes about an hour. If the EEG is done while you are sleeping, it usually takes about 3 hours.