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.
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.