Double dissociation between effects of prism adaptation and virtual-tilted environment on straight-ahead and verticality perception

The ego-centred straight-ahead (SA) and gravity-centred visual vertical (VV) constitute important reference frames for perception and action. Surprisingly, clinical studies have shown a strong association between SA and VV representational biases after right hemispheric stroke. It was therefore proposed that the brain might build 3D representations referenced to the body in the horizontal plane, and to gravity in the frontal plane. Here, we tackled this hypothesis in 37 right-handed healthy individuals (median age=20 years old, 23[62%] females), by testing whether two sensorimotor stimulations, prism adaptation (PA) and tilted virtual-reality (VTR) immersion, could bias both SA and VV orientations in a congruent, co-dependent fashion. Each participant underwent a session of right or left PA (optic shift: ±10°) and VTR (tilt: ±18°). Pre- and post-test SA and VV orientations were measured in darkness, by asking participants to point straight-ahead and to adjust a luminous line to the vertical, respectively. VV was also measured during VTR. As expected, we observed strong effects of PA on SA (ANOVA, np2=0.42), and of VTR on VV (np2=0.93), but only during the immersion. In contrast, PA had no effects on VV (p=.091, np2=0.09) and VTR had no effects on SA (p=.66, np2=0.01). Altogether, these results suggest that the brain represents space by using a mosaic of ego-centred and gravity-centred 2D coordinate systems, rather than a “hybrid” 3D coordinate system combining the “straight-ahead” and “straight-above”. Furthermore, the absence of long-lasting VTR effects might represent a break to the clinical use of VTR for impaired spatial orientation.