What does autonomic arousal tell us about locomotor learning?

Walking onto a stationary sled previously experienced as moving induces locomotor aftereffects (LAE, or "broken escalator phenomenon"). This particular form of aftereffect can develop after a single adaptation trial and occurs despite subjects being fully aware that the sled will not move. Here, we investigate whether such strong LAE expression may relate to arousal or fear related to instability during the gait adaptation process. Forty healthy subjects were allocated to three sled velocity groups; SLOW (0.6 m/s), MEDIUM (1.3 m/s), or FAST (2.0 m/s). Subjects walked onto the stationary sled for five trials (BEFORE), then onto the moving sled for 15 trials (adaptation or MOVING trials) and, finally, again onto the stationary sled for five trials (AFTER). Explicit warning regarding sled status was given. Trunk position, foot-sled contact timing, autonomic markers (electrodermal activity [EDA], ECG, respiratory movements) in addition to self-reported task-related confidence and state/trait anxiety were recorded. Trunk sway, EDA, and R-R interval shortening were greatest during the first MOVING trial (MOVING_1), progressively attenuating during subsequent MOVING trials. A LAE, recorded as increased gait velocity and trunk sway during AFTER_1, occurred in both MEDIUM and FAST sled velocity groups. The amplitude of forward trunk sway in AFTER_1 (an indicator of aftereffect magnitude) was related to EDA during the final adaptation trial (MOVING_15). AFTER_1 gait velocity (also an indicator of aftereffect magnitude) was related to MOVING_1 trunk sway. Hence, gait velocity and trunk sway components of the LAE are differentially related to kinematic and autonomic parameters during the early and late adaptation phase. The finding that EDA is a predictor of LAE expression indicates that autonomic arousal or fear-based mechanisms can promote locomotor learning. This could in turn explain some unusual characteristics of this LAE, namely its resistance to explicit knowledge and its generation with just a single adaptation trial.