Background: An electroencephalogram (EEG) is an accepted method in neurophysiology with a wide application. Different types of brain rhythms indicate that simultaneous activity of the brain cortex neurons depend on the person's mental state. Method: we have focus on reviewing the existing literature pertaining to changes of the brain's bioelectrical activity that recorded from the scalp in different conditions such as cognition and some mental disorders. Result: The frequency of brain waves may indicate sleep, consciousness, cognition, and some mental disorders. Slow brain waves are seen in some conditions such as sleep, coma, brain death, depression, autism, brain tumors, obsessive-compulsive disorder (OCD), attention deficit hyperactivity disorder (ADHD), and encephalitis, while rapid waves are generally reported in conditions such as epilepsy, anxiety, posttraumatic stress disorder (PTSD), and drug abuse. Conclusion: Increase in the EEG rhythm is a marker of high brain activity that leads to high degrees of consciousness, while slow waves are suggestive of less brain activity. The pattern of EEG rhythm can be an indicator of some mental disorders, too.
Keywords: Brain activity; Cognition; Consciousness; Electroencephalogram; Mental Disorders.