Concomitant motor responses facilitate the acquisition of multiple prior distributions in human coincidence timing

The brain optimizes timing behaviour by acquiring a prior distribution of target timing and integrating it with sensory inputs. Real events have distinct temporal statistics (e.g. fastball/slowball in ball sports), making it vital to acquire multiple prior distributions. In previous studies, participants acquired two prior distributions by assigning different types of motor responses or motor effectors to each prior. However, in daily tasks, different types of motor responses or effectors cannot always be selected for each target state. Here, we demonstrate that concomitant motor responses (CMRs) can facilitate multiple-prior acquisition. The non-CMR group made timing responses using only their dominant hand, irrespective of the prior distributions (short/long interval), whereas the CMR group selectively added a non-dominant hand response concomitantly to the dominant hand response for one of the priors. The CMR group acquired the two independent priors more quickly, and the divergence between the acquired priors was greater. Facilitation of multiple-prior acquisition was also observed with concomitant vocalization, indicating that this effect is not limited to bimanual interactions. These results demonstrate behavioural contexts that facilitate multiple-prior acquisition while using an identical type of motor response and effector, which can be effective in utilizing Bayesian estimation in daily life.