Electroencephalograph detects five main categories or types of brain waves based on frequency and amplitude:

- Delta waves (less than 4 Hz) - occur during deep sleep

- Theta waves (4 to 7 Hz) - associated with drowsiness or deep meditation

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- Alpha waves (8 to 13 Hz) - present during relaxation with eyes closed

- Beta waves (14 to 30 Hz) - engage when alert, attentive or actively thinking

- Gamma waves (above 30 Hz) - involved in high cognition functions like memory retrieval

The electrodes placed on the scalp pick up these fluctuations, which are amplified, processed and then displayed as line tracings or numerical values on a computer monitor. EEG technicians can then analyze the brain wave patterns for abnormalities.