Methadone addicts and non-addict controls were tested before and after receiving up to 10 mg of methadone on simple visual reaction time tests and on a vigilance type visual attention test. Addicts were faster than controls on pre-drug testing, although there were no pre-drug differences between groups on the attention task. Addicts maintained faster reaction times than controls even when money was offered as an incentive for speed. Additional methadone did not affect addict performance on any of the tasks. Methadone slowed control reaction times in a dose-related fashion. No significant attention decrements were seen after methadone in controls. Visual reaction time differences between addicts and controls cannot be attributed to group differences in motivation or ability to attend. Slowing of reaction time with acute dose of methadone in controls cannot be attributed to the effect of the drug on attention. An hypothesized drug-induced decrease in visual sensitivity with acute dose in controls and a drug-induced increase in visual sensitivity with chronic dose in addicts can account for the presented data.
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.
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