The main types of EEG waves are alpha, beta, theta and delta waves. Alpha waves are the most prominent component of the EEG. They are most marked in the parietooccipital area of the scalp when the person is awake, quite and resting with eyes closed. They disappear on opening the eyes and on attentive mind. They disappear entirely during deep sleep. They are fairly regular pattern of waves at a frequency of 8 - 13 / sec and an amplitude of 50 - 100 pV. The mean peak alpha frequency is 10.2 Hz and decreases in old age due to decreased cerebral perfusion leading to decreased cerebral metabolic rate. Frequencies of alpha rhythm are also decreased in conditions like low blood glucose level, low body temperature, low level of adrenal glucocorticoids and high arterial partial pressure of CO2.If there is a consistent difference of 1 Hz or more in alpha frequency between two cerebral hemispheres, the side of lower frequency is likely to be involved in pathological process.
Beta waves have frequency more than 13 cycles per second and may be as high as 25Hz. They have lower voltage than alpha waves. They are frequently recorded from the parietal and frontal region. They are seen during tension or CNS activation. When attentive to external stimulus or thinking hard about anything, the a wave is replaced by (3 rhythm. This transformation is known as EEG arousal. The seniles are found to have significantly less alpha or more beta activity that the young adult group. In infants, there is a fast beta like activity in EEG and occipital rhythm is slow 0.5-2/sec pattern. Barbiturates induce beta activity typically at a frequency of 18-24 Hz.
Theta waves have frequency between 4 - 8 Hz and have larger amplitude than alpha waves. They are seen in parietal and temporal region in children. They are seen in emotional stress in adults particularly during disappointment and frustration, and also occur in many brain disorders. The incidence of transient theta component is about 30% in an alert adult. Amplitude of theta component is greatest at 6 - 9 month (up to 150 pV when eyes are closed) of age. The theta component of EEG often accentuates during crying of children. Theta components persist into adult life in 10 - 15% of normal subject.
Delta waves have frequency of less than 3 Hz. They are seen in deep sleep (stage III and IV NREM) and in infancy. When they occur in awake state, they indicate serious organic brain disease.