Time in motion: effects of whole-body rotatory accelerations on timekeeping processes.

The ability of effectively representing time ensures the efficiency and accuracy of sensory and motor processing. It is well documented that in still observers, subjective time varies in response to variations of external sensory inputs. However, it is still poorly understood how inertial inputs, which enable coding of body displacements in space, affect timekeeping processes in a dynamic agent. Here, we investigated the effects of rotatory body accelerations on the reproduction of an acoustic isochronous pacing rhythm. In a first experiment, healthy participants performed a finger tapping task in which responses were either synchronized to the rhythm (Synchronization), or performed in absence of the rhythm following its withdrawal (Continuation). Both tasks were performed in presence and absence of sinusoidal acceleratory rotations along the vertical head-body axis. We found that the representation of the target frequency varied continuously as a function of periodic variations of vestibular-proprioceptive information. However, the effects on Synchronization and Continuation were opposite in directionality: increases in velocity were associated to increases in Continuation tapping rate (indicating a subjective shortening of the target interval), and decreases in Synchronization tapping rate. This was due to different temporal delays with which body motion affected tapping rate generation in these two conditions. A second control experiment, which lacked a representational component of time, confirmed that body displacements in Experiment 1 had indeed affected an internal timekeeper, and not motor responses triggered by its operation. A third control experiment, procedurally identical to Experiment 1 with the exception of an increased displacement frequency, allowed us to establish that Continuation tapping rate varied anticipatorily with respect to body motion, while Synchronization tapping rate varied with a delay in response to body movements. The observed consistent directionality in timing error, can be considered an adaptive response of internal timing mechanisms to body movements in space, where greater rates of displacement prompt accelerated timed responses.