Separating decision and motor contributions to behavioral biases induced by manipulating stimulus probability

Stimulus probability manipulations typically increase the proportion of responses associated with the more likely stimulus and reduce response times, especially when stimulus discriminability is low. Sequential sampling models primarily attribute these asymmetries to shifts in the starting point of evidence accumulation, but possible modulations at the motor execution stage and their links to upstream processing remain poorly understood. To address this gap, we conducted electromyographic recordings of muscle activity during a random dot motion task with manual responses, varying stimulus probability and stimulus discriminability. We then used a computational framework linking decision and motor processes, the gated cascade diffusion model, to account for behavior and muscle activity. Stimulus probability strongly modulated the time from stimulus onset to response-related muscle activation: on correct trials, muscle onsets occurred earlier for expected than unexpected stimuli, whereas this pattern reversed on incorrect trials. By contrast, it had little impact on the time from muscle activation to response completion. Stimulus probability also modulated subthreshold muscle activations: when the first activation occurred in the response channel, accuracy was higher for expected than unexpected stimuli, whereas this effect reversed when the first activation occurred in the opposite channel. The model accounted for these effects through an additive bias to the rate of evidence accumulation that favored expected stimuli, and an evidence-independent urgency signal at the motor preparation level that was engaged earlier for the response alternative linked to expected stimuli. These analyses advance our understanding of how stimulus probability influences processing across the decision-motor cascade.