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 brainwave 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")