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Meaghan McManus, Laurence R Harris; When Gravity Is Not Where It Should Be: Effects On Perceived Self-Motion. Journal of Vision 2019;19(10):237. doi: https://doi.org/10.1167/19.10.237.
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When gravity cues are unavailable, visual information is weighted more strongly (Harris et al., 2017 Microgravity 3:3). This suggests that if gravity information were ignored visual information might similarly be enhanced. When immersed in an upright visual scene, supine or prone viewers often experience a visual reorientation illusion (VRI) where they feel upright, and visually-induced self-motion (vection) is enhanced (McManus & Harris, 2016 IMRF). VRIs may reflect a conflict resolution between visual and non-visual uprights in which vision becomes dominant. Here we investigated the connection between VRIs and vection using virtual environments that varied the level of conflict between the cues. Targets were simulated in an Oculus Rift (CV1) at 10–80m. When the target disappeared, participants were virtually accelerated either through a hallway with strong orientation cues or a star field with no cues to orientation, while standing, supine or prone. They indicated when they reached the remembered target location. The hallway was more effective than the starfield at evoking a VRI (hallway 65%, starfield 50%) especially when prone (prone 82%, supine 47%). In the hallway there was a main effect of posture on target distance (p< 0.01) and vection was enhanced more while prone than when supine (p< 0.01). A separate ANOVA found no effect of posture in the starfield (p< 0.05). We conclude that change in visual weighting (indicated by perceived travel distance) is most likely to be caused by conflict between a prone posture and strong visual cues to upright. This conflict is most likely to evoke both a VRI and enhanced vection. If the posture/vision conflict is reduced by bending only the head while leaving the body upright, enhanced self-motion is also experienced only with the head prone (nose towards the ground) (McManus & Harris, 2017 IMRF).
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