Virtual reality improves embodiment and neuropathic pain caused by spinal cord injury

Abstract

Objective: To investigate changes in body ownership and chronic neuropathic pain in patients with spinal cord injury (SCI) using multisensory own body illusions and virtual reality (VR).

Methods: Twenty patients with SCI with paraplegia and 20 healthy control participants (HC) participated in 2 factorial, randomized, repeated-measures design studies. In the virtual leg illusion (VLI), we applied asynchronous or synchronous visuotactile stimulation to the participant's back (either immediately above the lesion level or at the shoulder) and to the virtual legs as seen on a VR head-mounted display. We tested the effect of the VLI on the sense of leg ownership (questionnaires) and on perceived neuropathic pain (visual analogue scale pain ratings). We compared illusory leg ownership with illusory global body ownership (induced in the full body illusion [FBI]), by applying asynchronous or synchronous visuotactile stimulation to the participant's back and the back of a virtual body as seen on a head-mounted display.

Results: Our data show that patients with SCI are less sensitive to multisensory stimulations inducing illusory leg ownership (as compared to HC) and that leg ownership decreased with time since SCI. In contrast, we found no differences between groups in global body ownership as tested in the FBI. VLI and FBI were both associated with mild analgesia that was only during the VLI specific for synchronous visuotactile stimulation and the lower back position.

Conclusions: The present findings show that VR exposure using multisensory stimulation differently affected leg vs body ownership, and is associated with mild analgesia with potential for SCI neurorehabilitation protocols.

Figure 1. Experimental setups

(A) In the virtual leg illusion (VLI) paradigm, the participant sits in a wheelchair and wears a head-mounted display (HMD) and headphones. The experimenter simultaneously strokes the lower or upper part of the participant's back and the corresponding part of the dummy leg. The camera films dummy legs from the distance and angle that corresponds to the participant's first-person viewpoint, and the real-time video recording is projected onto the HMD. Thus, the participant sees touch cues applied to the virtual legs while being touched on the back. (B) In the full body illusion (FBI) paradigm, the participant sits in a wheelchair and wears headphones and an HMD. A video camera, standing 2 meters behind, films the participant's back, while the experimenter is applying tactile stimulation to the participant's back with a wooden stick. The real-time (delayed for 800 ms in asynchronous condition) video is projected onto the HMD. The participant thus sees his or her own virtual body projected in front and being touched with the stick, while at the same time feels being touched on the back.

Figure 2. Virtual leg illusion (VLI) results

(A) Mean ipsatized ratings of the VLI questionnaire items: significant main effects of synchrony were found for the ratings of ownership, illusory touch, and referred touch. Significant main effect of group was found for the ratings of ownership. (B) Mean differences in neuropathic pain between baseline and postcondition ratings in the VLI. (C) Exponential decaying relationship between the time since lesion and ratings of ownership (C.a) or referred touch (C.b) in The VLI: significant relationships between the illusion and time since lesion were found for synchronous stimulation of lower back, but not upper back. Async = asynchronous; HC = control group; L back = lower back; Sync = synchronous; SCI = spinal cord injury group; U back = upper back. SCI HC *p < 0.05, **p < 0.010, ***p < 0.001. Error bars show standard error of the mean.

Figure 3. Full body illusion (FBI) results

(A) Mean ipsatized ratings of the FBI questionnaire items: significant main effects of synchrony were found for the ratings of ownership and illusory touch, but not for control items. The differences between the groups were not significant. (B) Mean differences in neuropathic pain between baseline and postcondition ratings for synchronous and asynchronous condition in the FBI: significant main effect of synchrony was found for the ratings of ownership, illusory touch, and referred touch. Significant main effect of group was found for the ownership ratings. Async = asynchronous; HC = control group; SCI = spinal cord injury group; Sync = synchronous. *p < 0.05, ***p < 0.001. Error bars show standard error of the mean.