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The perceptual upright results from the multisensory integration of the directions indicated by vision and gravity as well as a prior assumption that upright is towards the head. The direction of gravity is signalled by multiple cues, the predominant of which are the otoliths of the vestibular system and somatosensory information from contact with the support surface. Here, we used neutral buoyancy to remove somatosensory information while retaining vestibular cues, thus "splitting the gravity vector" leaving only the vestibular component. In this way, neutral buoyancy can be used as a microgravity analogue. We assessed spatial orientation using the oriented character recognition test (OChaRT, which yields the perceptual upright, PU) under both neutrally buoyant and terrestrial conditions. The effect of visual cues to upright (the visual effect) was reduced under neutral buoyancy compared to on land but the influence of gravity was unaffected. We found no significant change in the relative weighting of vision, gravity, or body cues, in contrast to results found both in long-duration microgravity and during head-down bed rest. These results indicate a relatively minor role for somatosensation in determining the perceptual upright in the presence of vestibular cues. Short-duration neutral buoyancy is a weak analogue for microgravity exposure in terms of its perceptual consequences compared to long-duration head-down bed rest.
The perceived distance of self motion induced in a stationary observer by optic flow is overestimated (Redlick et al., Vis Res. 2001 41: 213). Here we assessed how different components of translational optic flow contribute to perceived distance traveled. Subjects sat on a stationary bicycle in front of a virtual reality display that extended beyond 90deg on each side. They monocularly viewed a target presented in a virtual hallway wallpapered with stripes that changed colour to prevent tracking individual stripes. Subjects then looked centrally or 30, 60 or 90° eccentrically while their view was restricted to an ellipse with faded edges (25 x 42deg) centered on their fixation. Subjects judged when they had reached the target’s remembered position. Perceptual gain (perceived/actual distance traveled) was highest when subjects were looking in a direction that depended on the simulated speed of motion. Results were modeled as the sum of separate mechanisms sensitive to radial and laminar optic flow. In our display distances were perceived as compressed. However, there was no correlation between perceptual compression and perceived speed of motion. These results suggest that visually induced self motion in virtual displays can be subject to large but predictable error.