APA Dictionary of Psychology

in electroencephalography, a type of regular brain wave with a frequency of 4 to 7 Hz. Theta waves occur during REM sleep in nonhuman animals, Stage 2 NREM sleep in humans, and the drowsy state prior to sleep onset in newborn infants, adolescents, and adults. Such waves are also recorded in trances, hypnosis, and daydreams. The hippocampus and frontal lobes are sources of theta activity. Also called theta rhythm.