Methamphetamine-dependent participants showed more variable response timing during a vigilance task, and variability was associated with poorer functional outcomes. Average response speed alone did not capture this pattern.
Morgan and colleagues compared 35 methamphetamine-dependent participants with 55 comparison participants. They analyzed fluctuations within a person’s responses across a continuous performance task, alongside broader neuropsychological and everyday-function measures. This was observational human research, not a controlled methamphetamine dose experiment.
Response variability was elevated in the dependent group after accounting for specified covariates, including mean reaction time, global cognitive performance and HIV status. The magnitude of the group difference was comparable across trial blocks. Within the dependence group, greater variability was associated with executive dysfunction, psychomotor slowing, more recent use and poorer driving-simulator and laboratory functional performance, as well as cognitive complaints.
A stable average can conceal occasional slow or inconsistent responses. Conversely, a variability score does not establish that a specific lapse occurred during a real event. The measured functional associations do not create a validated collision probability or a universal diagnostic rule.
The cross-sectional design cannot establish whether drug use caused variability, whether preexisting characteristics contributed to both, or whether reducing variability would improve daily outcomes. Use history and recency estimates are not direct measures of brain concentration. No blood, urine or oral-fluid threshold was validated for this outcome. Individual assessment should examine the actual distribution of responses, test conditions, effort, sleep and medical history, rather than inferring consistency from a single average or impairment from a drug-history label. The study supports a more complete description of performance, with its observational limitations retained.
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:10.1016/j.psychres.2014.07.008.