Delta waves are seen in

Delta waves include all the waves of the EEG with frequencies less than 3.5 cycles/sec, and they often have voltages two to four times greater than most other types of brain waves. They occur in very deep sleep, in infancy, and in persons with serious organic brain disease. They also occur in the coex of animals that have had subcoical transactions in which the cerebral coex is separated from the thalamus. Therefore, delta waves can occur strictly in the coex independent of activities in lower regions ofthe brain. Delta waves also occur during deep slow-wave sleep, which suggests that the coex then is mainly released from the activating influences of the thalamus and other lower centers. Slow-wave sleep is divided into four stages. In the first stage, a stage of light sleep, the voltage of the EEG waves becomes low. This stage is broken by "sleep spindles" (i.e., sho spindle-shaped bursts of alpha waves that occur periodically). In stages 2, 3, and 4 of slow-wave sleep, the frequency of the EEG becomes progressively slower until it reaches a frequency of only one to three waves per second in stage 4; these waves are delta waves.Ref: Guyton and Hall textbook of medical physiology, 13th edition.