A PGO signal was an electrical signal transmitted from the humanoid brain as they drifted into REM sleep. This signal could be received by electrodes, which aided the physician in monitoring the patient. In 2367, when Deanna Troi was induced into REM sleep so she could telepathically communicate with the Tyken's Rift being, Doctor Beverly Crusher verbally commented to herself and her nurse that the PGO signals were steady. (TNG: "Night Terrors" )