Space-time relativity in self-motion reproduction.

Experiments on reproducing imposed self-motion showed that not only final distance or angle of motion, but also the temporal profile are reproduced. Reproduction errors have been attributed to sensory inputs, inaccurate memorization of the motion variable, or motor errors. However, another possible source of error has so far been neglected. The internal time base for path integration or movement memorization may be distorted and thus not reflect physical time. Because additional cognitive load was previously shown to affect subjective estimation of duration, we used a dual-task paradigm during either the stimulation or reproduction phase of three different movement reproduction tasks. We asked subjects 1) on a rotating chair to reproduce imposed passive whole body rotations by controlling the chair with a joystick, 2) on a treadmill to actively reproduce locomotion with respect to the treadmill, and 3) while blindfolded to reproduce a previously walked straight trajectory. The cognitive load changed the distance of reproduced self-motion by about 25% depending on whether the mental task was performed while experiencing or reproducing the motion. Although imposed velocity was reproduced accurately in all conditions, reproduced movement duration was affected in the same way as distance. This result implies that for the perception of distance traveled, perceptual space and time are closely interrelated. The findings are consistent with shared processing of temporal and spatial information. A computational model of motion reproduction including a discrete path integrator is proposed that is able to explain the experimental results within one coherent framework.