A hyperencephalogram is a 23rd century medical analysis of brain-wave activity, measured as a brain circuitry pattern, or BCP. It may be the natural descendant of the 20th century electroencephalogram, or EEG. The BCP is considered a unique characteristic for every individual, akin to fingerprints, making that individual's hyperencephalogram an important part of his or her medical record.
An individual's hyperencephalogram can be altered if interfered with by another resident brain-wave activity, such as an invading telepathic or noncorporeal lifeform. Mira Romaine's hyperencephalogram was altered when she became possessed by invading Zetarians, and her hyperencephalogram was instrumental in diagnosing this. (TOS: "The Changeling", "The Lights of Zetar").