Stop-Signal Reaction Time Largely Reflects Sensory and Motor Delays
The stop-signal reaction time (SSRT) is often interpreted as capturing people’s cognitive ability for rapidly and flexibly inhibiting prepotent actions when not desirable anymore. Its calculation is designed to circumvent the effect of strategy on baseline speed in the hope to specifically isolate the speed of signals conceived as reactive, top-down and inhibitory. However, the ability to act upon any signal, be it by producing or withholding a response, also always depends on i) when the information becomes available within decision areas, i.e. sensory delay, and ii) how long actions take to be executed (because only actions still in the planning stage can be withheld, and only executed actions are observable behaviourally). In line with (i), SSRT is clearly influenced by stop-signal contrast. In line with (ii), manual SSRT is also clearly longer than saccadic SSRT. In recent work (https://www.biorxiv.org/content/10.1101/2023.02.20.529290v1), we showed that the sum of (i) and (ii), i.e. non-decision time, is directly measured by the dip onset time (the earliest time point where the signal-absent and signal-present RT distributions depart). Importantly, this measure is immune to top-down factors, including whether the task is to stop or ignore the signal. Predictably, we show that individual differences in manual and saccadic SSRT are correlated with dip onset time (all r > 0.5 across datasets). This result is consistent with individual differences in SSRT largely reflecting sensory and/or motor influences, rather than being a pure measure of top-down inhibition speed. Alternatively, there may also be genuine correlations within the population between, on the one hand, the speed of bottom-up and/or motor signals and, on the other hand, the speed of top-down signals. Either way, this result calls for a reconsideration of past conclusions drawn from the use of SSRT (554 articles according to pubmed on the 29th of March, 2023), in particular studies reporting changes in SSRT in relation to clinical conditions, drugs, age, personality traits, general intelligence, brain structure, brain function, genes or performance on other tasks. Our results open the possibility that within and between-individual differences in SSRT, so far interpreted as differences in top-down control abilities, largely reflect variations in sensory delay and/or motor output time.