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

Abstract

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, is 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 symmetry 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.

Tactile stimuli affect long-range correlations of stride interval and stride length differently during walking

“…This effect was attributed to enhanced proprioceptive gain in response to the reduced tactile feedback. In a subsequent study (Eikema et al 2016), we demonstrated that such effects may be reversed by the presence of simulated optic flow during split-belt learning. It is unlikely that such specific effects which require feedback of alternate treadmill belt velocities could be affected by tactile stimulation at any random part of the body.…”

“…Therefore, in the current study, we attribute the reduced LRC during preferred walking gait at the supra-threshold tactile stimulation of 250 Hz and 17.5 db to increased proprioceptive gain resulting in greater degrees of freedom and increased flexibility. However, availability of external visual cues limits any changes in proprioceptive gain and the same stimulation (250 Hz and 17.5 db) does not demonstrate enhanced transfer effects in a split-belt adaptation task (Eikema et al 2016). It is possible that 17.5 db is a threshold level at or above which vibro-tactile stimulation is perceived as noise requiring alternate sensory gain changes (e.g., proprioception) in the absence of external visual cues.…”

“…While a case can be made for sensory stimulation on other parts of the body affecting gait dynamics, our intention was to specifically target the tactile stimulus that is felt under the foot during gait. Stimulation at other locations may lead to similar effects but these locations will probably be those that have a role to play in gait control, for example, proprioceptive stimulation on the neck, thighs, or Achilles tendon (Ivanenko et al 2000a, b), vestibular stimulation on the mastoid process (Chien et al 2016), or tactile stimulation below the feet (Mukherjee et al 2015; Eikema et al 2016). We showed that continuous foot perturbation through vibratory tactors enhanced split-belt locomotor adaptation (Mukherjee et al 2015).…”

Incremental Visual Occlusion During Split-Belt Treadmill Walking Has No Gradient Effect on Adaptation or Retention

“…Adaptation and transfer were more robust when visual feedback was occluded during split-belt treadmill walking (Torres-Oviedo & Bastian, 2010). Possibly, visual feedback occlusion might induce a sensory reweighting, resulting in higher proprioceptive gain, due to an increased reliance on body-based cues (Eikema et al, 2016).…”

Using a Split-belt Treadmill to Evaluate Generalization of Human Locomotor Adaptation

“…However, this is not an exhaustive list. There are numerous possible protocol modifications, some of which have already been shown to affect adaptation and/or after-effects, including the addition or deprivation of sensory stimuli 26,38,39,40 , the rate of acceleration of treadmill belts at the beginning of split-belt trials 27 , the practice structure 29 , and providing feedback during adaptation 34,41 . After-effects following split-belt walking are very robust and have been replicated in numerous studies (e.g.…”