Modulating the internal model of verticality by virtual reality and body-weight support walking: A pilot study.

BACKGROUND AND OBJECTIVE: The study aimed at inducing a visual vertical (VV) bias by immersion in a virtual tilted room (VTR, visual cues), then testing the effect of 30% body-weight support walking (BWSW, somaesthetic cues) to correct this bias.

METHODS: We included 20 healthy participants (median age 54 years; 12 females) who wore the Oculus-Rift® Head Mounted Display to produce the virtual reality and generate the VV. VV (8 trials) was tested at baseline, then in 3 postural conditions (walking, sitting and BWSW), by 2 visual conditions (darkness and VTR), according to a pseudo-randomized blocked design. The VTR was tilted 18° clockwise. Data for 3 participants with virtual reality sickness were discarded, and those for 17 participants underwent non-parametric statistical analysis by 2 main criteria: VV and head orientation.

RESULTS: The VTR induced a pronounced tilt of the vertical toward the tilted side under the baseline condition (median 11.4° [Q1-Q3 6.1-13.4]; P<0.01), with a large effect size (r=0.88). The effect was systematic, with great inter-individual variability (2-17°), and was similar under every postural condition (P<0.001), with a post-effect lasting 6min and suppressed under BWSW. In darkness, VV was more upright during BWSW than sitting (P<0.05), with a medium effect size (r=0.49). The VTR induced a slight head tilt of median 3.3° [2.8-5.9] toward the tilted side under every postural condition (P<0.001), with a large effect size (r=0.87). In darkness, the head was upright only at baseline and under BWSW.

CONCLUSION: Being immersed in a tilted environment induces a powerful bias in verticality perception (11°). Contrary to our hypothesis, BWSW did not attenuate the effect induced by the VTR, probably because of the power of this effect. However, BWSW was the only postural condition able to suppress post-effects induced by the VTR, thereby leading to the head and VV oriented upright. BWSW may improve verticality representation, presumably by bringing augmented information about the direction of the Earth vertical. These findings represent an avenue for rehabilitation of patients with postural disorders caused by a wrong verticality representation. Technological improvements will be necessary to attenuate the virtual reality discomfort.