Rat movements reflect internal decision dynamics in an evidence accumulation task

Perceptual decision-making involves multiple cognitive processes, including accumulation of sensory evidence, planning, and executing a motor action. How these processes are intertwined is unclear; some models assume that decision-related processes precede motor execution, whereas others propose that movements reflecting ongoing decision processes occur before commitment to a choice. Here we combine two complementary methods to study the relationship between decision processes and the movements leading up to a choice. The first is a free-response pulse-based evidence accumulation task, in which stimuli continue until choice is reported, and the second is a motion-based drift diffusion model (mDDM), in which movement variables from video pose estimation constrain decision parameters on a trial-by-trial basis. We find that the mDDM provides a better fit to rats' decisions in the free-response accumulation task than traditional drift diffusion models. Interestingly, on each trial we observed a period, before choice, that was characterized by head immobility. The length of this period was positively correlated with the rats' decision bounds, and stimuli presented during this period had the greatest impact on choice. Together these results support a model in which internal decision dynamics are reflected in movements and demonstrate that inclusion of movement parameters improves the performance of diffusion-to-bound decision models.NEW & NOTEWORTHY In this study we combine a novel pulse-based evidence accumulation task with a newly developed motion-based drift diffusion model (mDDM). In this model, we incorporate movement parameters derived from high-resolution video data to estimate parameters of the model on a trial-by-trial basis. We find that this new model is an improved description of animal choice behavior.