Reinforcement and error feedback differentially impact motor exploration during locomotion

Reinforcement and error feedback are known to encourage motor exploration and learning. Studies of the upper limb have shown that reaches that are not rewarded lead to greater exploration, because the nervous system utilizes knowledge of movement variability to update aim towards a successful "reinforced" action. The current study investigated how reinforcement and error feedback influences motor exploration while walking and its impact on upright balance control.

Twenty-four healthy young subjects walked on an instrumented treadmill with a 180° virtual reality screen presenting feedback on their step length or step width. Subjects were instructed to match a target step length or width. Visual error feedback displayed their left foot step length or width as a black line within a target area. Reinforcement feedback displayed a gray target box that would turn blue when their left foot stepped within the box. We calculated lag-1 autocorrelations to assess exploratory behavior. Linear models of ankle roll and step placement were applied to assess whether the recruitment of balance mechanisms differed across feedback conditions.

Both baseline walking and reinforcement feedback conditions led to greater motor exploration than error feedback for step length, but not for step width. Error feedback yielded corrective behavior, narrowing in on the target center regardless of target direction. Lag-1 values during reinforcement feedback were similar to baseline walking for both target directions. Analysis of the balance mechanisms showed that step width feedback changed balance strategy use. Center of mass was less predictive of ankle roll for the step width target conditions than at baseline.

The regulation of step length follows similar trends to the regulation of upper body reaching behaviour under both error and reinforcement feedback. However, maintaining balance leads to reduced exploration for step width targets, and altered balance mechanism use in response to feedback.