Specific oculomotor deficit after acute methadone. II. Smooth pursuit eye movements.

Changes in smooth pursuit eye tracking of targets moving sinusoidally in horizontal and vertical planes before and after up to 10 mg oral methadone were measured electrooculographically in nontolerant nondependent humans. Methadone depressed the gain of horizontal tracking movements at most frequencies tested (0.2 - 1.6 Hz) without changing target-eye phase relationships. Lack of change in target-eye cross-correlation functions after methadone and examination of individual records indicate that gain reduction was due to the eye failing to follow the target to the full extent of target excusion (+5 degrees from eyes straight ahead view). Possible mechanisms and the practical consequences of methadone action on smooth pursuit were discussed.