Reaction time
Reaction time is how long a person takes to respond to a stimulus. Simple reaction time, the minimal time needed to respond to a stimulus, is a basic measure of processing speed.
What it means in practice
Francis Galton first measured simple reaction times in the 19th century and reported visual latencies below 190 ms in young people [1]. Recent large studies reported much longer times that differ between laboratories, partly because computer hardware and software introduce timing delays to the measurement [1].
In a calibrated computer test of 1,469 community adults aged 18 to 65, mean simple reaction time was 231 ms, or 213 ms after correcting for hardware delays [1]. It rose with age by 0.55 ms per year but did not differ by sex or education [1]. The same study estimated that detecting the stimulus took about 131 ms, a figure that did not change with age, and concluded that age-related slowing came mainly from slower motor output [1].
Reaction time is also the raw measure behind the psychomotor vigilance test (PVT), which is very sensitive to sleep deprivation [3]. The vigilance entry covers what the PVT shows under sleep loss.
Why it matters when reading nootropics research
Reaction time is counted in milliseconds, and timing delays from hardware and software help explain why results differ between laboratories, so the details of measurement matter [1]. Check what equipment was used and which summary measure the trial reports.
The choice of measure changes what a study can detect. In a study of 74 healthy adults aged 22 to 45, PVT measures based on lapses and on response speed (mean 1/RT) showed large effects of sleep loss, while the widely used mean and median reaction time had low to moderate effect sizes, influenced by extreme values [2]. So a trial that reports only a mean reaction time may miss changes that other summaries would show.
A trial might report a faster mean reaction time with a supplement. The questions to ask are how much faster than placebo, on which measure, in rested or sleep-deprived people, and whether reaction time was a primary outcome. On NootropicsWork, reaction time is graded separately from processing speed, which covers broader cognitive speed tasks.
References
- Woods DL, Wyma JM, Yund EW, et al. Factors influencing the latency of simple reaction time. Front Hum Neurosci. 2015.
- Basner M, Dinges DF. Maximizing sensitivity of the psychomotor vigilance test (PVT) to sleep loss. Sleep. 2011.
- Lim J, Dinges DF. Sleep deprivation and vigilant attention. Ann N Y Acad Sci. 2008.