With a rate of 8-12 cycles per second alpha waves have the highest amplitude with an average of 50 microvolts. It is observed in the occipital, frontal and parietal regions of the cortex and cannot occur without the connection of RAS . It is prominent in darkness and/ or when eyes are closed and abolished with visual stimulation or mental concentration. Beta waves are usually found in parietal and frontal regions. It ranges between 18 and 60 (maximum) cycles per second with amplitude varying between 5-10 microvolts. Human scalp emits two types of beta waves- ß I and ß II. The frequency of ß I is twice that of a waves. While ß I disappears during mental activity ß II is apparent during stress and intense activation of CNS . Theta waves are primarily found in the parietal and temporal regions and are prominently observed in children between the ages of 2-5 years. Its rate ranges between 4-8 cycles per second with amplitude of 10 microvolts. With a rate of minimum 0.5-3.5 cycles per second and amplitude that varies between 20-200 microvolts, delta waves are rarely found in normal adults during working hours. It is observed during deep sleep, organic brain diseases, hypoxia and hypoglycemia. It is understood as a byproduct of separation of the cortex and RAS .

The table given below illustrates a comparative review of the four brain waves.

EEG has diagnostic application for epileptic cases. Before being replaced by CT scans and MRI , it was also used as first-line diagnosis for tumors and other brain disorders. It is useful as a n on-invasive technique and indicates the active brain area, but cannot precisely indicate where the signal came from. The EEG records electrical activities only from the cortical surface close to the skull and hence reflects the functional state of about 35% of the cortex only. The above description is of the normal EEG pattern. Minor differences have been observed in the EEG records of 5-7% of normal population but they have no clinical significance. EEG is also age-dependent; younger the child slower the activities. Elderly people have slower pattern indicating that with the increase in age the EEG pattern becomes slower.