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Richard T. Dyde, Michael R. Jenkin, Heather L. Jenkin, Jim E. Zacher, Laurence R. Harris; The role of visual background orientation on the perceptual upright during microgravity. Journal of Vision 2006;6(6):183. doi: 10.1167/6.6.183.
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The perceptual upright (PU) — the orientation in which an object is most easily and naturally recognized — is determined by a combination of the orientation of the body, the visual background, and gravity. PU can be assessed by identifying a character the identity of which depends on its orientation (the Oriented Character Recognition Test: OCHART, Dyde et al. VSS 2004. J. Vision, 4(8), 385a). Using OCHART we measured the influence of the orientation of the visual background on the PU in the fronto-parallel plane under conditions where gravity was irrelevant (when the character was presented orthogonal to gravity, with the subject lying supine); or not present (during exposure to microgravity created during parabolic flight). When supine in 1g the influence of the background on the PU was reliably greater than when the observer was upright in 1g. In microgravity the influence of the background on PU was reliably less than in the equivalent 1g state; curiously a similar reduction relative to the 1g condition was also found during the hyper-gravity phase of parabolic flight. These perceptual changes are consistent with an increase in the use of the body as a reference frame when gravity is changed. The effects of microgravity in the fronto-parallel plane cannot be simulated by simply arranging gravity to be orthogonal to that plane by lying supine.
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