Faster responding can coexist with poorer response control. In a controlled interaction study, d-amphetamine shortened correct-response times while increasing commission errors on a sustained-attention task.
The experiment included 12 healthy volunteers, seven men and five women, in a double-blind, placebo-controlled crossover design. Across four sessions, participants received placebo, 20 mg d-amphetamine, 100 mg riluzole, or the combination.
Researchers measured cardiovascular responses, plasma cortisol, subjective effects and performance on the Sustained Attention to Response Test. That task requires maintaining responses while withholding them on designated trials.
D-Amphetamine increased heart rate, blood pressure and plasma cortisol and produced typical stimulant-like subjective effects. On the attention task, correct responses became faster, but commission errors increased significantly.
Riluzole also increased commission errors without the same speeding of reaction time. At the tested dose, it did not block the typical physiological and subjective responses to d-amphetamine. Thus, the findings did not support a straightforward prediction that reducing glutamate release would suppress those stimulant responses.
A faster correct response is one outcome; successfully withholding an inappropriate response is another. Calling the overall pattern an improvement would omit the additional errors. Calling it uniform slowing would also be wrong.
Plasma cortisol was an endocrine response, not a measure of amphetamine concentration in the brain. Subjective stimulation likewise did not provide a direct substitute for performance.
The study was small and used single doses in healthy volunteers. Its pharmacological mechanism remained uncertain, and its findings do not establish effects of chronic treatment, other doses or other populations.
The paper illustrates why speed, accuracy and inhibitory failures should be reviewed together. It does not validate the task as a roadside diagnostic test or establish how much functional change a particular drug concentration would cause in an individual.
Related reading: What a controlled driving study can establish.