Processing speed
Processing speed is the speed with which a person can carry out mental processing operations. Timed coding tasks such as the Digit Symbol Substitution Test are a common measure of it.
What it means in practice
One account is Salthouse's processing-speed theory of adult age differences in cognition [1]. It proposes that increased age in adulthood is associated with a decrease in the speed of many processing operations, and that this slowing impairs cognitive functioning [1].
The theory names two mechanisms [1]. Under the limited time mechanism, relevant operations cannot be finished when processing is slow; under the simultaneity mechanism, the products of early processing may no longer be available when later processing is complete [1]. Evidence offered for the theory includes considerable shared age-related variance across many different speed measures [1].
A common test is the Digit Symbol Substitution Test (DSST), in which a person matches symbols to numbers using a key at the top of the page [3]. The score is the number of correct symbols within the allowed time, usually 90 to 120 seconds [3]. The DSST is among the most commonly used tests in clinical neuropsychology [3]. That review's medical writing support was funded by two drug makers, Takeda and Lundbeck, and the author's company consults for pharmaceutical companies [3].
Why it matters when reading nootropics research
The DSST can be sensitive without being specific. The same review, whose writing support was funded by Takeda and Lundbeck, notes that the DSST has low specificity for determining exactly which cognitive domain has been affected [3]. It also notes that good DSST performance requires intact motor speed, attention and visuoperceptual functions, so a faster score can reflect quicker thinking, quicker hands or both [3].
Motor speed is a real part of the picture. In one large study of simple reaction time, age-related slowing came mainly from slower motor output, while the time to detect the stimulus did not change with age [2]. So when a trial reports better processing speed, check what the task required, including how much it depended on hand speed.
Some researchers also count simple reaction time as a basic measure of processing speed [2]. On NootropicsWork, processing speed is graded separately from reaction time and psychomotor speed, even though the tasks overlap, so reaction-time results are graded under Reaction time.
References
- Salthouse TA. The processing-speed theory of adult age differences in cognition. Psychol Rev. 1996.
- Woods DL, Wyma JM, Yund EW, et al. Factors influencing the latency of simple reaction time. Front Hum Neurosci. 2015.
- Jaeger J. Digit Symbol Substitution Test: The Case for Sensitivity Over Specificity in Neuropsychological Testing. J Clin Psychopharmacol. 2018.