Can visual acceleration evoke a sensation of tilt?

Under the microgravity of the International Space Station, many of the normal processes that determine the perceptual upright on Earth are disrupted. For example, somatosensory cues are absent and an applied physical linear acceleration can provide an artificial "gravity" reference. Here, we hypothesized that visual linear acceleration could also be interpreted as an orientation cue in microgravity. Using virtual reality, we subjected twelve astronauts experiencing long-duration exposure to microgravity to visually simulated accelerating linear self-motion along a virtual corridor at 0.8 m•s (0.083 G) for 16s. They then adjusted a virtual ground plane to indicate whether they had changed their perceived orientation. Control experiments used visually simulated linear self-motion at a constant velocity and control experiments on Earth mirrored the experiments conducted in microgravity in both upright and supine postures. Contrary to our hypothesis, no significant perceptual tilts were induced on Earth or in microgravity. However, we did replicate earlier results that both microgravity exposure (in comparison to on Earth) and a supine posture (in comparison to a sitting upright posture) were associated with higher variability in judgements of upright. Our experiments failed to demonstrate that exposure to visual acceleration can evoke a sense of tilt in a stationary observer in the dark, either in microgravity or on Earth.N = 209.