Electroencephalography (EEG) is utilized to see the electro-physiological dynamics of the brain through the placement of metal electrodes on the head without any surgical intervention. Electro-physiological dynamics are visible on the screen in the form of zigzags. These dynamics provide information about cognitive skills and disorders. They can be viewed during the sleep or wakefulness.
Electrical fields are created while electrochemical signals are transmitted between nerves. When millions of these signals come together, electrical fields accumulate and become strong to be imaged from outside of the head via specific methods. Hans Berger discovered these electrical fields produced by the brain in the 19th century and worked on associating them with some cognitive and behavioral functions and disorders.
The most commonly observed value during the normal-abnormal separation of the waves is their frequencies. There are four waves classified based on frequencies: delta, theta, alpha, beta. Waves with 3 or less waves are called delta, waves with 4-7 frequencies theta, those with 8-12 frequencies alpha, and those having over 13 frequencies beta. Propagated from the skull, these waves are compared to the images that are supposed to be, considering the age and population of the person, and the abnormal ones are detected. A wave frequency of 8 or above released from a person who is awake is considered normal, while a frequency of 7 and less is considered normal for children and a person who is sleeping.
Delta waves are the biggest type of wave and are slow. They are observed during the sleep in each age group. Imaging a delta wave in adults who are awake is considered abnormal. They can be imaged in a specific center of the brain, or in a scattered form.
Theta waves are imaged during the sleep at all ages. Imaging a theta wave in adults who are awake is considered abnormal. They are also slow like the delta waves.
Alpha waves are observed in all age groups, particularly in adults. They scatter from the right and left parts of the brain in a rhythmical manner. Particularly when eyes are closed, they are expected to be imaged in harmony with the rear head parts. The alpha activity disappears when eyes are open, when under stress and when the attention increases. In patients who are insensitive to stimulants, they equally spread from the front and rear parts of the head.
Beta waves are observed in all age groups. They are generally symmetrical and apparent in the front part of the head. Drugs like barbiturate and benzodiazepine increase the beta wave.