Effect of prism adaptation and virtual-tilted environment on straight-ahead and verticality perception in healthy...

The ego-centred straight-ahead (SA) and gravity-centred visual vertical (VV) constitute important reference frames for perception and action. Clinical studies have shown a strong association between SA and VV representational biases following right hemispheric stroke, suggesting that the brain builds 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; 23 [62%] females) by testing whether prism adaptation (PA) and immersion in a tilted virtual environment (VTE) could bias both SA and VV orientations in a congruent, co-dependent manner. Each participant underwent a session of right or left PA (optic shift: ± 10°) and VTE (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 VTE immersion. For PA, the expected post-effect on SA orientation (changes≈3.5°; ANOVA, p < 10^-6, n_p² = 0.43) did not extent to VV orientation (changes < 0.4°, p = .10, n_p² = 0.07). For VTE, we found a strong effect on VV orientation during the immersion (changes≈12°; p < 10^-6, n_p² = 0.93), and up to 4-5 min after VTE arrest, with a mild post-effect (≈1.5°). No VTE post-effects was found on SA orientation (changes < 0.8°, p = .66, n_p² = 0.01), even just after VTE arrest (< 1°). These patterns of dissociations suggests that verticality and straight-ahead representations were independent, which supports the view that the brain represents space by using dissociated and dynamic 2D coordinate systems.