The psycho-tricorder was a Starfleet tricorder specifically programmed for a psychologist's use in analyzing a patient. This version of the tricorder scanned specific brain wave patterns during questioning of the patient, and was helpful in diagnosis and treatment of mental ailments and disorders. It may have possessed a hypnosis-assist subroutine, as well as subroutines for lie-detection and amnesia analysis. The device was normally operated by a medical technician while the attending psychologist directed the patient.
A psycho-tricorder and technician were once requisitioned by Kirk and McCoy to help analyze the apparently amnesiac Scotty when he became a suspect in several murders on Argelius II. The technician who was to operate the device – Lieutenant Karen Tracy – was killed either by someone possessed by Redjac or by the entity taking physical form, before she could complete the scans. (TOS: "Wolf in the Fold")