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Michiteru Kitazaki, Isanobu Kurose; Postural sway from combined optical flow of radial and lateral motions. Journal of Vision 2003;3(9):548. doi: 10.1167/3.9.548.
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© ARVO (1962-2015); The Authors (2016-present)
When radial motion is presented, observers incline in the opposite direction to heading direction. For instance, when an optical flow simulates self-motion to left-front, observers inclines to right-back. It indicates a correlation of posture control and heading perception. To test if the visual control of posture is based on perceived heading, we employed a combined optical flow of radial and lateral motions as stimulus, because it elicits interesting illusion (eg Royden & Hildreth, 1996). When expanding motion is superimposed by lateral motion, heading is biased in the direction of lateral motion, though the averaged flow indicates focus-of-expansion shifting to the opposite direction. Thus, if the control of posture is based on perceived heading, observer should incline to the opposite direction to lateral motion. Observer's linear motion (104cm/s; forward /backward) going through 3-D cloud of dots was simulated, and projected on a screen (228×182cm [50x40deg], 4090dots visible). Superimposed lateral motion (9cm/s; leftward/rightward) was on front-parallel plane (182×182cm [40×40deg], 2000dots). We manipulated the depth of lateral motion with binocular disparity: far, near or same as the radial flow which was at zero disparity. Subjects observed stimuli at 260cm viewing distance. In Expt1, subjects just fixated eyes on a center marker. In Expt2, subjects were asked to pay attention to either radial or lateral motion while fixating eyes. Posture was measured by a force plate at 60Hz. Observers inclined in the direction of the same- and near-depth lateral motions in Expt1. When they paid attention to lateral motion, postural sway in the direction of lateral motion was enhanced and found in all depth conditions in Expt2. These results are inconsistent with the prediction based on heading perception, and suggest that the visual control of posture is processed in different way from heading perception, and utilizes averaged flow or individual motions in parallel.
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