Optic flow improves adaptability of spatiotemporal characteristics during split-belt locomotor adaptation with tactile stimulation

Human locomotor adaptation requires feedback and feed-forward control processes to maintain an appropriate walking pattern. Adaptation may require the use of visual and proprioceptive input to decode altered movement dynamics and generate an appropriate response. After a person transfers from an extreme sensory environment and back, as astronauts do when they return from spaceflight, the prolonged period required for re-adaptation can pose a significant burden. In our previous paper, we showed that plantar tactile vibration during a split-belt adaptation task did not interfere with the treadmill adaptation however, larger overground transfer effects with a slower decay resulted. Such effects, in the absence of visual feedback (of motion) and perturbation of tactile feedback, are believed to be due to a higher proprioceptive gain because, in the absence of relevant external dynamic cues such as optic flow, reliance on body-based cues is enhanced during gait tasks through multisensory integration. In this study, we therefore investigated the effect of optic flow on tactile-stimulated split-belt adaptation as a paradigm to facilitate the sensorimotor adaptation process. Twenty healthy young adults, separated into two matched groups, participated in the study. All participants performed an overground walking trial followed by a split-belt treadmill adaptation protocol. The tactile group (TC) received vibratory plantar tactile stimulation only, whereas the virtual reality and tactile group (VRT) received an additional concurrent visual stimulation: a moving virtual corridor, inducing perceived self-motion. A post-treadmill overground trial was performed to determine adaptation transfer. Interlimb coordination of spatiotemporal and kinetic variables was quantified using symmetry indices and analyzed using repeated-measures ANOVA. Marked changes of step length characteristics were observed in both groups during split-belt adaptation. Stance and swing time symmetries were similar in the two groups, suggesting that temporal parameters are not modified by optic flow. However, whereas the TC group displayed significant stance time asymmetries during the post-treadmill session, such aftereffects were absent in the VRT group. The results indicated that the enhanced transfer resulting from exposure to plantar cutaneous vibration during adaptation was alleviated by optic flow information. The presence of visual self-motion information may have reduced proprioceptive gain during learning. Thus, during overground walking, the learned proprioceptive split-belt pattern is more rapidly overridden by visual input due to its increased relative gain. The results suggest that when visual stimulation is provided during adaptive training, the system acquires the novel movement dynamics while maintaining the ability to flexibly adapt to different environments.