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Philip M. Jaekl, Michael R. Jenkin, Richard T. Dyde, Laurence R. Harris; Perceptual stability during active and passive head translation: variations with direction. Journal of Vision 2003;3(9):492. doi: 10.1167/3.9.492.
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The world in which we live normally appears perceptually stable as we move around during both active movements and when moving passively, as for example when traveling as a passenger. To achieve such stability, the visual motion experienced has to be compatible with that expected to be associated with the self motion. This in turn requires knowledge about the self motion. During active head movement, information is derived from an efference copy of motor commands as well as vestibular and other proprioceptive sources. Efference copy cues are not available during passive motion. Are there differences between the perception of visual stability under active and passive motion? In previous studies we measured the amount of visual motion needed for the world to appear stable during active head motion and found that ∼1.5 times more visual movement was required than was geometrically necessary. Naso-occipital motion was closer to veridical than other directions (Jaekl et al., 2002, J. Vis. 2(7), 508a). The present study measured the visual motion needed to appear stable during passive motion. Subjects were translated passively at 0.5 Hz while sitting on a cart manually pulled against a powerful spring. Subject motion was naso-occipital, inter-aural or in between. Subjects wore a head-mounted display and were positioned in the centre of a virtual spherical world, radius 1m, which was updated in response to head movement monitored by a mechanical tracker. Subjects varied the ratio between head and image motion until the display appeared perceptually stable. During passive translation, even more visual motion was required to ensure the appearance of moving within an earth stable environment than when motion was active, often needing as much as twice the geometrically required value. Passive naso-occipital motion was again closer to requiring the geometrically correct amount. The experiments help establish the role of efference copy in the perception of self motion.
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