An electroencephalogram, or EEG, is a medical test that records tiny electrical signals produced by the brain. It matters because brain cells communicate using electrical activity, and changes in that activity can reveal important clues about health. EEG machines are commonly used to help diagnose seizures, sleep disorders, brain injuries, and some neurological conditions.
The test is noninvasive because the electrodes sit on the scalp and do not send electricity into the brain.
Understanding Medical Technology: EEG Machines
An EEG signal is not the activity of one neuron. It is mainly the combined effect of many thousands or millions of brain cells changing their electrical state at nearly the same time. These cells are usually in the outer folded layer of the brain, called the cortex.
A single cell produces a signal far too small and local to detect through the skull. When groups of cells line up and work in a similar rhythm, their tiny voltage changes add together.
The skull, skin, and fluids between the brain and scalp weaken and blur the signal. This is why EEG gives excellent timing information but less exact information about the deep location of activity.
The electrodes are placed in a standard pattern so recordings from different people can be compared. Their labels relate to areas of the head, such as front, side, middle, and back. Each channel shows the voltage difference between two selected electrodes, or between one electrode and a reference.
Clean contact is essential. Technicians measure electrode impedance, which describes how much the skin contact resists electrical current.
Thick hair, dry skin, loose wires, or poor gel contact can make a trace noisy. The machine then makes the weak signal larger, removes some unwanted frequencies, converts it into numbers, and stores it for review.
Not every wave on an EEG comes from the brain. Blinking creates a large signal near the forehead. Eye movements, jaw clenching, swallowing, talking, sweating, heartbeats, and nearby electrical equipment can produce artifacts.
A trained technician notes when these events happen. During a recording, a person may be asked to open and close their eyes, breathe deeply for several minutes, or watch a flashing light.
These activities can change normal rhythms or, in some people, bring out unusual patterns. Sleep is often especially useful because some seizure related electrical changes appear more clearly during drowsiness or sleep than during wakefulness.
Doctors do not diagnose from one jagged line alone. They compare patterns across many channels, check whether a pattern repeats, and connect it with the person’s symptoms and medical history. A seizure can produce sudden, organized bursts of activity, but a normal EEG between events does not always rule out epilepsy.
Longer recordings can improve the chance of capturing an event. Students should remember that frequency describes how quickly a pattern repeats, while amplitude describes the size of its voltage change. A faster rhythm has a shorter period.
Brain wave bands are useful labels, but they are not simple mood meters. For example, alpha activity often becomes stronger when a relaxed person closes their eyes, yet the meaning of a pattern depends on its location, timing, shape, and the person’s state.
Key Facts
- EEG stands for electroencephalogram, which is a recording of brain electrical activity over time.
- Scalp electrodes detect voltage differences, often in microvolts, where 1 microvolt = 1 x 10^-6 V.
- Frequency is the number of wave cycles per second, measured in hertz: f = cycles / time.
- Common EEG bands include delta 0.5 to 4 Hz, theta 4 to 8 Hz, alpha 8 to 13 Hz, beta 13 to 30 Hz, and gamma above about 30 Hz.
- The time for one wave cycle is the period: T = 1 / f.
- EEG signals are amplified, filtered, digitized, and displayed as brain-wave traces from different electrode locations.
Vocabulary
- EEG
- An EEG is a test that records electrical activity from the brain using electrodes placed on the scalp.
- Electrode
- An electrode is a small sensor that detects voltage changes at the skin surface.
- Brain wave
- A brain wave is a repeating pattern of electrical activity measured from groups of neurons.
- Frequency
- Frequency is the number of wave cycles that occur each second, measured in hertz.
- Artifact
- An artifact is unwanted signal in an EEG recording caused by sources such as eye movement, muscle activity, or loose electrodes.
Common Mistakes to Avoid
- Thinking the EEG machine reads thoughts. EEG records patterns of electrical activity from large groups of neurons, not specific ideas or memories.
- Assuming scalp electrodes put electricity into the brain. Standard EEG electrodes only detect voltage changes and do not stimulate the brain during recording.
- Confusing amplitude with frequency. Amplitude describes signal size, while frequency describes how many cycles occur each second.
- Ignoring artifacts in the trace. Eye blinks, jaw movement, poor electrode contact, and electrical noise can create signals that look like brain activity but are not.
Practice Questions
- 1 An EEG wave completes 20 cycles in 4 seconds. What is its frequency in hertz, and which common EEG band does it fall in?
- 2 A brain wave has a frequency of 10 Hz. What is its period in seconds, using T = 1 / f?
- 3 During an EEG recording, a patient clenches their jaw and several channels suddenly show large irregular signals. Explain why a technician should mark or remove this section before interpreting the brain waves.