High-human acuity of speed asymmetry during walking

1Despite its central role in the proper functioning of the motor system, sensation has been less studied than motor output in sensorimotor adaptation paradigms. This deficit is probably due to the difficulty of measuring sensation: while motor output has easily observable consequences, sensation is by definition an internal variable of the motor system. In this study we asked how well can subjects estimate relevant environmental changes inducing motor adaptation. We addressed this question in the context of walking on a split-belt treadmill, which allows subjects to experience distinct belt speeds for each leg. We used a two-alternative forced-choice perceptual task (2AFC) in which subjects report which belt they thought to be moving slower. We characterized baseline accuracy in this task for healthy human subjects, and found 75% accuracy for 75 mm/s speed differences. Additionally, we used a drift-diffusion model of the task that could account for both accuracy and reaction times. We conclude that 2AFC tasks can be used to probe subjects estimates of the environment and that this approach opens an avenue for investigating perceptual deficits and its relation to motor impairments in clinical populations.