Responding more slowly does not necessarily mean that handling conflict is selectively impaired. A 2019 study of adults after prolonged methamphetamine abstinence found slower overall responses alongside several preserved task effects. The distinction matters when a broad impairment label is applied to a specific cognitive operation.
A selected matched sample
Bensmann and colleagues initially recruited 32 former methamphetamine consumers and 32 individually matched controls. Eight former consumers were excluded, including four who did not perform above chance; the corresponding matched controls were also removed. The final sample contained 24 participants in each group.
The former-consumer group reported approximately 7.2 years of prior use and an average 31.9 months of abstinence, ranging from 1.5 to 120 months. These were retrospective histories, not a uniform abstinence interval. The sample partly overlapped with an earlier study and should not be counted as an entirely independent cohort from that work.
What does a flanker task measure?
Participants selected responses while surrounding information either supported or conflicted with the target. The flanker effect compares conflicting with matching trials. The Gratton effect examines how the preceding trial’s conflict changes the current conflict effect.
The investigators also measured post-error slowing: a change in response speed after a mistake. General response speed, conflict interference, adjustment after conflict and adjustment after errors were therefore separate outcomes.
Which result differed between groups?
Overall responses were slower in former consumers: approximately 369 milliseconds versus 351 milliseconds in controls, with p = 0.028. This was a general group effect, not evidence of a uniquely enlarged conflict cost.
The groups did not differ significantly in overall accuracy, the size of the flanker effect, the Gratton effect or post-error slowing. Complementary Bayesian analyses generally supported the absence of the tested group differences, rather than relying only on nonsignificant conventional tests.
Bayesian support remains conditional on the models and assumptions used. It is not proof that all possible differences are zero, nor a demonstration of equal real-world functioning across every domain.
Does this prove cognitive recovery?
No. The study did not follow the same participants from active use through prolonged abstinence. People who achieved longer abstinence and volunteered for research may differ from people who did not. The findings could reflect preserved function, recovery, selection or a combination.
Excluding participants who did not perform above chance also limits generalization to people with greater task difficulty. The reported null comparisons describe the retained sample; they should not be used to erase the excluded participants’ performance problems.
Why not compare short and long abstinence directly?
The authors explored a median split of the former-consumer group, but each subgroup contained only 12 people. They cautioned against strong conclusions because of limited power. That exploratory comparison cannot establish a minimum abstinence duration required for preserved control.
The study supports a precise statement: selected adults with prolonged abstinence showed general slowing without demonstrably larger conflict-related or post-error costs on this task. It does not establish that all cognition was unaffected, that past methamphetamine use caused the slowing, or that a particular individual was fit or unfit to drive.
Historical primary source
Bensmann W, Ernst J, Rädle M, Opitz A, Beste C, Stock AK. Methamphetamine Users Show No Behavioral Deficits in Response Selection After Protracted Abstinence. Frontiers in Psychiatry. 2019;10:823. doi:10.3389/fpsyt.2019.00823.
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