The difficulty in numerical computation impacts motor decisions in a Stop Signal task

The proper interpretation of environmental information is necessary for effective decision-making. The resulting cognitive burden may affect the entire process if interpretation is not instantaneous. In this study, we investigated how numerical distance (ND), a measure of cognitive demand in numerical comparisons, influences movement initiation and inhibition. To this end, 32 participants completed a novel numerical comparison Stop-Signal Task (NC-SST), in which the cognitive demand of each trial was manipulated by varying the ND between pairs of numbers in both Go and Stop signals. Participants were required to initiate or stop a movement if an upcoming number was higher or smaller than the one presented previously. Results showed that larger NDs (i.e., easier comparisons) facilitated faster and more accurate responses during movement initiation and enhanced stopping performance. Using a generalized drift-diffusion model, we found that drift rates increased with ND and were modulated by the spatial location of numerical stimuli, consistent with a left-to-right space number association. A generalized linear mixed-effects model further revealed that Go process parameters, particularly the drift rate, strongly predicted successful stopping and interacted with Stop ND and Stop signal delay (SSD). These findings demonstrate that higher cognitive load impairs both movement initiation and inhibition, and that motor decisions result from the integration of cognitive information onto perceptual features, extending the classical race model framework.