Concurrent locomotor adaptation and retention to visual and split-belt perturbations

Abstract

Gait asymmetry is a common symptom in groups with neurological disorders and significantly reduces gait efficiency. To develop efficient training for gait rehabilitation, we propose a novel gait rehabilitation paradigm that combines two distinct perturbation strategies: visual feedback distortion (VFD) and split-belt treadmill (SBT) walking. In SBT walking, spatiotemporal gait adaptation can be readily achieved, but it quickly fades after training. Gait adaptation to implicit VFD in an unconscious manner tends to persist longer, potentially due to a greater engagement of implicit learning during training. Thus, we investigated whether the combined strategies would lead to more effective changes in symmetric gait patterns with longer retention periods. We compared the retention of the preserved asymmetric pattern acquired by "implicit VFD+SBT walking" with "SBT-only walking" and with "SBT walking with conscious correction". In the implicit VFD+SBT walking, the speed of the two belts was gradually changed, the visual representation of gait symmetry was implicitly distorted, and no instructions were given to subjects except to watch the visual feedback. In the SBT walking with conscious correction, subjects were instructed to consciously correct their steps with the help of visual feedback while SBT walking. The SBT-only walking consisted of SBT walking with no visual feedback. After the 7-minute adaptation period, we removed the visual feedback and the split-belt perturbations, and we assessed the retention of the preserved asymmetric pattern while subjects continued walking for the 15-minute post-adaptation period. In a group of subjects who spontaneously showed visuomotor adaptation in response to the implicit VFD (16 out of 27 subjects), we found a greater retention rate during the implicit VFD+SBT walking trial than the SBT-only walking or the SBT walking with conscious correction trials. The implicit visual distortion paradigm delivered in an attention-independent (unconscious) manner can be utilized and integrated into SBT walking to improve the efficacy of symmetric gait adaptation by producing longer-lasting effects on the retention of a newly learned motor pattern.

Fig 1. Experimental setup showing an instrumented treadmill and visual feedback.

A: Three infrared cameras were placed behind to capture the two markers attached to the subjects’ shoes. B: Visual representation of subjects’ movement was captured using a motion capture system. The range of the right and left step lengths was mapped to the vertical bars. During the swing phase of a leg, the corresponding bar increases proportional to the step length and stops when a heel strike occurs for that leg. For visual distortion, the range of right step length mapped to the visual bar was gradually distorted.

Fig 2. Changes in step length symmetry.

A: Example of changes in step length symmetry resulted from the SBT-only trial and the implicit VFD+SBT trial obtained from a subject. B: Example of changes in step length symmetry resulted from the SBT-only and the conscious VF+SBT trials obtained from another subject. In all trials, visual distortion and split-bet perturbations were only used during the first 13 minutes (adaptation period). The intermittent horizontal lines indicate distortion or perturbation increments applied during those time periods. The horizontal axis shows time; the vertical axis shows the step length symmetry ratio between the actual left and right step lengths. The markers represent the mean step length symmetry value averaged over 30-second intervals at a given time.

Fig 3. Group results of averages step length symmetry over time.

A: Changes in the mean step length symmetry due to varying visual distortion and split-belt perturbation averaged across 16 subjects who showed visuomotor adaptation during the implicit VFD+SBT and SBT-only conditions. B: Changes in the mean step length symmetry of 11 subjects who did not show visuomotor adaptation for the same conditions. The error bars indicate the standard deviation of a reported mean step length symmetry for each level/time. The asterisks (*) were marked at time periods, where the induced step length symmetry values were significantly different between different adaptation conditions (p<0.05).

Fig 4. Changes in the mean step length symmetry averaged across 12 subjects during the conscious VF+SBT and the SBT-only conditions.

The asterisks (*) were marked at time periods, where the induced step length symmetry values were significantly different between two conditions (p<0.05).

Fig 5. Change in step length symmetry during the post-adaptation period.

Group means step length symmetry values (19 subjects for the SBT-only, 16 subjects for the implicit VFD+SBT, and 12 subjects for the conscious VF+SBT) were normalized by the initial values (100%). Displaying standard deviation and significant differences were ignored in this plot (refer to Figs 3A and 4).