Ambulatory searching task reveals importance of somatosensation for lower-limb amputees

Abstract

The contribution of somatosensation to locomotor deficits in below-knee amputees (BKAs) has not been fully explored. Unilateral disruption of plantar sensation causes able-bodied individuals to adopt locomotor characteristics that resemble those of unilateral BKAs, suggesting that restoring somatosensation may improve locomotion for amputees. In prior studies, we demonstrated that electrically stimulating the residual nerves of amputees elicited somatosensory percepts that were felt as occurring in the missing foot. Subsequently, we developed a sensory neuroprosthesis that modulated stimulation-evoked sensation in response to interactions between the prosthesis and the environment. To characterize the impact of the sensory neuroprosthesis on locomotion, we created a novel ambulatory searching task. The task involved walking on a horizontal ladder while blindfolded, which engaged plantar sensation while minimizing visual compensation. We first compared the performance of six BKAs to 14 able-bodied controls. Able-bodied individuals demonstrated higher foot placement accuracy than BKAs, indicating that the ladder test was sensitive enough to detect locomotor deficits. When three of the original six BKAs used the sensory neuroprosthesis, the tradeoff between speed and accuracy significantly improved for two of them. This study advanced our understanding of how cutaneous plantar sensation can be used to acquire action-related information during challenging locomotor tasks.

Lower-Limb Amputees Adjust Quiet Stance in Response to Manipulations of Plantar Sensation

“…Furthermore, PNS perceived as arising from locations similar to those of vibration stimuli caused similar directional shifts, indicating that the internal model treats perturbations “elicited” by PNS similarly to perturbations “detected” by mechanoreceptors in the intact foot sole. This adds to the growing body of evidence that PNS provides useful information for balance control and movement planning ( Petrini et al, 2019a ; Charkhkar et al, 2020 ; Christie et al, 2020 ).…”

“…Similarities between responses to PNS and responses to vibration on the intact foot sole indicate that the sensorimotor system treats tactile inputs induced by PNS similarly to native tactile inputs. PNS sensory inputs can provide information about foot-ground contact by modulating PNS in response to readings from pressure sensors placed underneath the prosthetic foot ( Charkhkar et al, 2020 ; Christie et al, 2020 ). Our observations encourage future investigation of the dynamics of internal model responses to PNS feedback, such as threshold detection of changes during stance (e.g., acceleration akin to a slip, Richerson et al, 2003 ) and adaptation during tasks with and without feedback (e.g., Engels et al, 2019 ).…”

“…In place of the missing biological mechanoreceptors, this system provides specific tactile sensations that can be modulated by changing the stimulation delivered. Electrical current delivered to the nerves via individual electrode contacts of a high-density nerve cuff can be selectively tuned to elicit sensations of touch and pressure at discrete areas on the missing foot ( Charkhkar et al, 2018 , 2020 ; Christie et al, 2020 ). Our findings along with other reports suggest that activating remaining neural pathways in the residual limb of lower-limb amputees can provide sensory feedback from the missing limb that informs user behavior ( Clites et al, 2018 ; Petrini et al, 2019b ; Charkhkar et al, 2020 ).…”

“…Our findings along with other reports suggest that activating remaining neural pathways in the residual limb of lower-limb amputees can provide sensory feedback from the missing limb that informs user behavior ( Clites et al, 2018 ; Petrini et al, 2019b ; Charkhkar et al, 2020 ). Modulating evoked tactile sensations in response to foot-floor contact pressures enhances balance control during perturbations in the visual field and stance surface ( Charkhkar et al, 2020 ) and improves the tradeoff between speed and accuracy in an ambulatory searching task ( Christie et al, 2020 ). It can also decrease the number of falls when walking over obstacles ( Petrini et al, 2019b ).…”

Bionic limb replacement: an evolving concept in lower extremity reconstruction

“…Commercially available prostheses do not have volitional control, especially where it is needed, the ankle. We know that sensorimotor interruption after limb loss leads amputees to have reduced confidence in maintaining balance, increasing falls risk and bearing greater cognitive burden [69] . Humans combine sensory, visual, vestibular, cognitive, and proprioceptive inputs to their surrounding environment [70] .…”