Motor decision is influenced by the difficulty of solving a numerical comparison in a selective stop signal task

The environment's variability often requires us to assess whether an ongoing action should be completed or promptly interrupted in the event of an unforeseen circumstance. In experimental settings, these environmental interactions are simulated through selective versions of the Stop Signal Task (selective SST). These tasks require evaluating whether to cancel or not a planned movement based on the information provided by signals discriminated for the different perceptual characteristics. A theoretical model has been developed to explain the task's outcomes, wherein a Go and a Stop process engage in a race towards a common finish line. Within this framework, interpreting an event occurring during motor preparation as a countermanding input has been found to influence the race's dynamics. However, sometimes, deciding whether to ignore a stop signal requires going beyond simple perceptual processing and evaluating its impact on the ongoing decision. In our study, we investigated how the difficulty in the engaged cognitive operation for interpreting an external stimulus as a command to countermand ongoing action influences action inhibition. To do so, we designed a stimulusselective version of the SST, where 18 participants were required to engage in a selective finger movement or inhibition, based on a numerical comparison. They were instructed to make a comparison between two numbers presented in the opposite positions of the screen and lift the index finger if the number on the left side of the screen was higher or lift the middle finger for the opposite arrangement of the numbers. After a variable delay, randomly chosen between two possible values, in 40% of trials the higher number on the screen was replaced by a smaller number (Stop signal) or by a larger number (Ignore signal) than the target one. By manipulating the numerical distance between the numbers to be compared, whereas reaction time was longer when comparing close numbers such as 5 vs. 6 than distant numbers such as 5 vs. 9. This subsequently influenced both the probability of correctly stopping a programmed movement and the corresponding reaction time to the stop signal (SSRT). Specifically, we found a significantly lower proportion of correctly inhibited movements for shorter numerical distances compared to longer ones (two-way ANOVA, ps < .05), and a gradual decrease in SSRT with increasing numerical distance (one-way ANOVA, p < .05). Overall, our results suggest that cognitive operations need to be considered in understanding how action inhibition occurs in complex contexts.