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Meaghan McManus, Laurence R Harris; Contributions of vision, gravity, and the body during misestimations of orientation. Journal of Vision 2020;20(11):886. doi: https://doi.org/10.1167/jov.20.11.886.
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When immersed in an upright (relative to them) visual scene, supine or prone viewers can experience a visual reorientation illusion (VRI) where they actually feel upright. When people report a VRI, visually-induced self-motion (vection) is enhanced (McManus & Harris, 2019 VSS). This might be due to 1) higher visual weightings in individuals who report a VRI compared to those who do not, or 2) a misinterpretation of the vestibular cue as motion instead of tilt or 3) a greater sensitivity to visual-vestibular conflict.
Here we investigated the connection between VRIs and sensory weighting using virtual reality. Participant’s sensitivity to VRIs was measured over 1 minute where they continuously pressed a button if they perceived themselves as upright while lying supine (VRI) with an upright display. They were divided into VRI and non-VRI groups. The perceptual upright (PU) was then measured while sitting or lying on their side to obtain the weightings of vision, body, and gravity. Participants reported whether an ambiguous symbol in various orientations appeared as a “p” or “d” as the visual background orientation was varied. The PU was defined as midway between the orientations of maximum ambiguity and the weighting of each cue determined.
The weightings of vision (mean difference= 6.22%, SE= 5.89, p=0.301) and body (mean difference= 4.45%, SE= 7.32, p=0.55) did not differ between the VRI and non-VRI groups, however the VRI group had a significantly higher weighting of gravity (mean difference= 10.67%, SE= 4.23, p=0.03).
It appears that despite their reported orientation being more influenced by visual cues and enhanced vection, VRI-sensitive people’s perceptual upright is more influenced by gravity. This finding is counter to the conclusion by Howard and Hu (2001) who supposed that during a VRI participants must be ignoring the gravity vector and perhaps indicative of greater sensitivity to conflict.
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