Working-Memory Load Reveals More Than a Single Methamphetamine-Related Test Score

A working-memory imaging study found that people with problematic methamphetamine-use experience responded differently as task demands increased. Examining the response to increasing load can reveal information that a single score or one brain image does not.

Several measures defined the ability

Roy and colleagues compared 65 participants with problematic methamphetamine-use experience and 44 without such experience. They combined performance on an n-back task during functional MRI with working-memory tasks outside the scanner. This allowed a modeled dimension of working-memory ability rather than relying exclusively on one task.

Increasing demands produced different patterns

The methamphetamine-experience group had lower working-memory scores. Comparisons showed increasing frontoparietal activation and decreasing default-mode activation as n-back load rose. Those load-related patterns were attenuated in the methamphetamine group. These are associations in a cross-sectional human sample, not effects isolated by randomized drug administration.

Not every brain-performance relationship was significant

Within the methamphetamine group, stronger load-related frontoparietal activation was associated with better working-memory ability. The corresponding default-mode relationship was not significant. A group difference in a network therefore does not automatically establish that the network explains individual performance.

Selection and forensic interpretation

The paper notes that people excluded during imaging quality assurance were more likely to have a positive methamphetamine urine screen and worse performance on one working-memory measure. That selection can affect generalization. Urine detection is not a quantitative measure of current impairment, and an fMRI signal is not a chemical drug concentration. These data do not establish what a person could remember during an earlier event, or a specimen cutoff for that ability.

Related evidence

For a different perspective on interpreting stimulant evidence, see our analysis of the Swedish amphetamine concentration study. Its study population and outcome should remain separate from the present findings.

Primary research

Methamphetamine-related working memory difficulties underpinned by reduced frontoparietal responses. Original human research. doi:10.1111/adb.13444.