brain waves

Meaning: Brain waves are rhythmic electrical fluctuations produced by large groups of cortical neurons and recorded at the scalp by electroencephalography (EEG). They are classified by frequency: beta (about 13–30 Hz) in alert, active thinking; alpha (about 8–13 Hz) in relaxed wakefulness, often with eyes closed; theta (about 4–8 Hz) in light sleep; delta (below about 4 Hz) in deep, slow-wave sleep. Stage 2 sleep also shows sleep spindles and K-complexes. Fast, low-amplitude waves reflect neurons firing out of step (desynchronized). Slow, high-amplitude waves reflect more synchronized firing.

Equation: f = 1/T, where f is frequency in Hz (s⁻¹) and T is the period of one wave cycle in s.

On the MCAT: Expect an EEG trace or sleep-stage graph with a question asking which state or stage it shows. Moving from alert wakefulness to deep sleep, frequency falls and amplitude rises. Passages may also compare EEG with eyes open versus closed, or across the sleep cycle.

Common confusion: Students assume REM sleep must show slow waves because the person is asleep. REM shows fast, low-amplitude, beta-like activity, which is why it is called paradoxical sleep. Delta waves characterize stage 3 slow-wave sleep. Also, a large amplitude does not mean more brain activity. It reflects more synchronized firing.