Methamphetamine’s effects on perceived control did not follow a simple all-or-nothing pattern in a controlled experiment. A low intranasal dose improved a task-specific judgment of agency, while higher doses increased certain errors under difficult conditions without globally disrupting control judgments.
Kirkpatrick and colleagues studied ten adults with current methamphetamine use disorders in a double-blind, within-participant inpatient experiment. Four sessions tested placebo and intranasal methamphetamine doses of 12, 25 and 50 mg per 70 kg. The computerized task occurred 45 minutes after dosing.
Participants tried to catch falling targets with a mouse while avoiding distractors, then rated their control. Speed and programmed turbulence changed task conditions. The construct was perceived control of a computer action, not legal capacity, credibility or the ability to drive.
At slow speed without turbulence, all active doses increased ratings of control. With turbulence at slow speed, only the 12-mg dose reduced control ratings relative to placebo. Under the authors’ operational definition, that low dose improved discrimination between favorable and unfavorable control conditions.
The 12-mg dose also improved target hit rate. At 25 and 50 mg, commission errors increased under the slow-speed turbulence condition. Under fast-speed conditions, control ratings did not differ significantly between doses. Reporting only an improvement or only an impairment would conceal the dependence on dose, task conditions and endpoint.
These were controlled human drug administrations, not observational cases or animal experiments. A known intranasal dose is not a blood, urine or oral-fluid concentration and does not supply a specimen-based impairment threshold.
The study used one agency task and did not directly compare perceived control with an independent measure of actual control. Participants were experienced users, the sample included only ten completers, and single doses differed from repeated binge use. The authors discussed possible mechanisms, but those explanations were not independently demonstrated by this experiment.
The useful question is whether a particular opinion concerns task accuracy, awareness of control, subjective confidence or another construct. This study demonstrates that they should not be assumed to move together. It does not establish that methamphetamine leaves all judgment intact or that feeling in control proves accurate performance.
Our overview of selected human performance findings supplies broader context. This experiment adds a distinct question: how an action and the person’s judgment about that action can diverge.
Read the original investigation: doi:10.1007/s00213-007-1018-2.