Electroencephalograph, << ih `lehk` troh ehn SEHF uh luh graf, >> is an instrument used to measure and record the electrical voltages produced by neurons (nerve cells) in the brain. A recording of this electrical activity is called an electroencephalogram. Both electroencephalograph and electroencephalogram are abbreviated EEG. Doctors and neuroscientists use the electroencephalograph to study normal brain activity. They also use it to study abnormal brain states that are caused by injury, tumors, infection, or even death.
To record an electroencephalogram, medical personnel attach electrodes from the electroencephalograph to the patient’s scalp. The electroencephalogram is usually recorded on a long, moving chart paper using ink pens that oscillate (move back and forth) with changes in the brain’s electrical activity. When the patient is relaxed with the eyes closed, the oscillations normally form a pattern that repeats approximately 10 times a second. Such brain waves are called alpha waves. When the person is alert and concentrating, the alpha waves tend to disappear and are replaced by smaller and faster beta waves. When a person is in a deep sleep, very large and slow theta waves and delta waves occur.
One of the most common medical applications of the electroencephalograph is to diagnose and study epilepsy. In epilepsy, abnormal discharges of certain neurons cause excessive electrical activity that interferes with normal brain function, resulting in a seizure. The electroencephalograph is used to detect and locate the brain regions that are responsible for seizures.
