Two people can have similar average responses to conflicting information yet differ in how a preceding conflict changes the next response. A 2013 methamphetamine study separated those outcomes instead of treating “Stroop performance” as one undifferentiated score.
The comparison and the task
Salo and colleagues studied 30 abstinent participants with methamphetamine dependence and 30 controls using a Stroop variant during functional MRI. The methamphetamine group had been abstinent for at least three weeks. No drug was administered.
Congruent trials provided nonconflicting information; incongruent trials created response conflict. The investigators measured the immediate conflict effect and compared incongruent trials preceded by either another incongruent trial or a congruent trial.
The preceding trial mattered
Controls responded approximately 26 milliseconds faster to conflict trials when another conflict trial had preceded them. The methamphetamine group showed no such advantage, instead showing an approximately four-millisecond cost. The between-group adjustment comparison was significant.
In contrast, the groups did not significantly differ in their overall within-trial Stroop conflict effect. They also did not significantly differ in congruent or incongruent error rates. Both groups made more errors on incongruent trials.
These findings support a specific difference in adaptation across trials. They do not establish a deficit on every aspect of attention or a universal inability to respond to conflict.
The imaging result was also specific
Anterior cingulate activation related to immediate conflict was observed in both groups without a significant group difference. For trial-to-trial adjustments, selected prefrontal regions showed lower activation in the methamphetamine group.
A reduced contrast in an imaging analysis is not a direct measure of neuronal loss. The study did not demonstrate an individualized neural threshold separating impaired and unimpaired people.
Why the design limits causal claims
The analysis included participants from an earlier preliminary report: 12 methamphetamine participants and 16 controls. The expanded study should not be counted as an entirely independent replication of that sample.
The groups were not matched on nicotine and cannabis-use histories. Adjustment for education or reading-based ability did not eliminate the main adaptation result, but statistical adjustment cannot exclude every pre-existing or exposure-related difference.
Because the study examined abstinent participants, it did not measure acute dose effects, driving performance or the cause of behavior during an earlier event. Nonsignificant correlations with drug-use variables do not prove that exposure history is irrelevant.
The useful distinction
Ask whether an expert’s claim concerns immediate conflict, adaptation to previous information, accuracy or an imaging contrast. Results from one endpoint should not be substituted for another. This historical study’s contribution is precisely that those endpoints did not all yield the same answer.
Primary scientific source
Salo R, Fassbender C, Buonocore MH, Ursu S. Behavioral regulation in methamphetamine abusers: An fMRI study. Psychiatry Research: Neuroimaging. 2013;211(3):234–238. doi:10.1016/j.pscychresns.2012.10.003. Historical observational human imaging study.
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