An electroencephalograph was a type of medical device which involved the attaching of electrodes to the scalp of the patient and was used to measure electrical activity produced by the brain. While the device was pioneered in the 20th century, a version of the device was still in use aboard Federation starships in the mid-23rd century. In 2266, Doctor Leonard McCoy of the USS Enterprise used an electroencephalograph to study Dr. Simon Van Gelder's brain and detected highly abnormal levels of delta wave activity. (TOS novelization: Dagger of the Mind)