Lost in Space: graviceptive biasing of visual perception

Abstract

Our brain receives a series of sensory snapshots of the external world, which it must integrate to provide a description of the Scenes around us. Vestibular inputs monitor changes in the position of the body relative to the environment. Here we tested thehypothesis that the vestibular system contributes to bridging the gap between successive visual snapshots of the external world. Accordingly, if gravitational signals provided by the vestibular organs cannot be aligned with those from vision, altered perceptual experiences may occur. This might underlie perceptual errors reported by pilots and astronauts exposed to altered gravitational forces. We investigated the contribution of vestibular-gravitational signals to the process of updating perception of a series of visual scenes. Ten participants were seated facing outwards on a short arm human centrifuge (SAHC) platform, which simulated 1 +Gz artificial gravity for ten minutes at head level. A visual judgement task was performed at normal gravity baseline and during 1 +Gz artificial gravity, in counterbalanced order. An environmental scene (Scene A) was followed after a short delay by a second scene (B) involving slight perspectival modification of Scene A. The scenes differed either in angular perspective (as if the participant had turned leftwards or rightwards during the delay) or in translational perspective (as if the participant had moved forward or backwards). Participants judged whether the implied viewpoint change between the first and second scene corresponded to a left/right-ward rotation (angular perspective), or to an approach/retreat (translational perspective). Artificial gravity influenced the perceived relation between the visual images: participants judged the second scene as significantly closer during 1 +Gz artificial gravity compared to a normal gravity baseline (t(9)=2.568, p=0.030). No differences were found in judgements of angular perspective. This dissociation rules out non-specific effects of artificial gravity or centrifugation which cannot readily explain this specificity. Our results support a vestibular-driven updating process in which gravity signals are computed to provide a dynamic description of the spatial position of the body relative to the external environment.