Adults with a history of methamphetamine dependence showed less adjustment after a conflicting Stroop trial, despite no significant group difference in the usual within-trial interference effect. The distinction concerns learning from the immediately preceding trial, rather than a blanket failure on the Stroop task.
Salo and colleagues studied 30 abstinent participants with lifetime methamphetamine dependence and 30 controls during functional MRI. Participants named stimulus colors under congruent and incongruent conditions. The investigators also compared conflicting trials according to whether the preceding trial had contained conflict.
The report extended an earlier investigation: data from 12 methamphetamine participants and 16 controls had previously appeared in preliminary findings. It should therefore be understood as an expanded dataset, rather than a wholly independent replication.
Controls responded about 26 milliseconds faster to a conflicting trial when another conflicting trial immediately preceded it. The methamphetamine group instead showed a small cost of about four milliseconds. This trial-to-trial adjustment differed significantly between groups, including analyses with demographic and reading-test covariates.
By contrast, the overall within-trial Stroop conflict effect did not differ significantly between groups. Neither did error rates within the congruent or incongruent conditions. Selected prefrontal regions showed lower activation associated with the adjustment contrast in the methamphetamine group. Those imaging results describe a particular comparison between trial sequences.
The groups differed in several demographic and substance-use characteristics. Statistical adjustment addresses specified measured variables, but cannot remove all confounding. Nicotine and cannabis histories were not matched, and participants were abstinent during testing. The design cannot determine acute drug effects or exclude vulnerabilities that preceded dependence.
The paper also contains an inconsistent description of the direction of a control-group brain–behavior correlation; that detail should not carry a clinical inference. The clearer behavioral result is the dissociation between ordinary interference and subsequent adjustment. Neither reaction times nor fMRI contrasts establish an individual’s past intoxication or driving impairment.
Read our analysis of a separate drug-cue conflict experiment.
Behavioral regulation in methamphetamine abusers: An fMRI study. doi:10.1016/j.pscychresns.2012.10.003.