Immersion in a virtual tilted environment strongly modulates perception and action with respect to gravity: a within-person randomized trial of healthy individuals

Background – Immersing individuals in a virtual tilted environment (VTE) could be a way to modulate verticality representation and its consequences on action with respect to gravity. The VIRGIL study involves a basic study of healthy individuals and a clinical trial of individuals showing post-stroke lateropulsion due to a biased internal model of verticality. Here we present the preclinical experimental findings for the basic study of healthy individuals. Methods - Twenty healthy individuals received 2 sessions of 45 min of VTE immersion (18° tilt), performed 1 to 2 days apart: one session of sitting tested the VTE on verticality representation (postural vertical [PV; primary outcome] and visual vertical [VV]), and one session of standing tested the VTE effect on the active body orientation with respect to gravity (measured with inertial captors) and asymmetry of weight-bearing on lower limbs (WB; measured by posturography). Session allocation was randomized, and the VTE side (left or right) was balanced between individuals. We assessed the discomfort associated with the VTE immersion. In this exploratory study, investigators and participants could not be masked to treatment. Results - VTE modulated verticality representation, with a transmodal tilt of PV and VV toward the side that the VTE was tilted: mean (SD) magnitude 3.8° (3.8°) for PV and median (Q1:Q3) magnitude 14.0° (12.0; 14.9) for VV (both p<10 ^-3 and large effect size). VTE also modulated WB on the ground, increasing the load on the lower limb toward the side that the VTE was tilted (mean +2.5% [0.7], p<10 ^-3 , large effect size). The active body orientation remained vertical during VTE immersion (p>0.05). The VTE discomfort was minimal in 75% of participants and null in 15%. Conclusion - In healthy individuals, VTE immersion strongly modulated perception and action with respect to gravity. In standing, the modulation of the WB is likely a response to the VTE to keep the body vertical. These findings open an avenue for the rehabilitation of lateropulsion after stroke.