A structural MRI study found a larger right putamen in active methamphetamine-dependent participants, with greater volume associated with fewer errors on a response-inhibition task. Brain size differences did not support a simple rule that enlargement proves either damage or improved overall function.
Jan and colleagues compared 17 active methamphetamine-dependent participants with 20 controls using structural MRI, voxel-based morphometry and regional volumetric analysis. This was observational human primary research. It measured anatomy and cognitive performance rather than administering a dose or directly examining tissue pathology.
Regional increases were observed in the right and left putamen and left nucleus accumbens, but the right-putamen finding remained significant after Bonferroni correction. Some cortical findings did not survive correction. Whole-brain, overall grey-matter, white-matter and cerebrospinal-fluid volumes did not differ significantly. A summary that presents every uncorrected regional observation as equally established would overstate the evidence.
Right-putamen volume was inversely associated with Go/No-go errors. The authors considered explanations including compensation, inflammation and other tissue processes. MRI volume alone cannot establish which mechanism produced the enlargement. Nor can a correlation show that making the region larger would improve performance.
The sample was small and groups differed in tobacco and cannabis use. Cross-sectional imaging cannot determine whether a difference preceded drug use, resulted from it or changed with abstinence. No brain-volume measure in this report establishes intoxication at a past event, and imaging is not interchangeable with blood or oral-fluid drug testing. The useful lesson is methodological: retain corrected and uncorrected findings separately, specify the task and avoid diagnosing an individual’s impairment or permanent injury from a group MRI association.
For a separate example of the limits of use-history comparisons, see our current-user driving study. The design and measured outcome determine which conclusions can be carried into an individual case.
Original investigation: doi:10.3390/brainsci2040553.