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Paul MacNeilage, Dora Angelaki; Visual and vestibular discrimination of heading azimuth and elevation for upright and side-down observers. Journal of Vision 2008;8(6):834. doi: 10.1167/8.6.834.
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Heading is the direction of observer translational relative to the stationary environment. For short duration movements in well-lit environments, heading may be estimated from both visual and vestibular cues. The visual cue to heading is the location of the focus of expansion in the optic flow field. The vestibular cue to heading is the direction of inertial acceleration signaled by the otoliths. Prior research has focused on visual discrimination of heading azimuth (heading in the horizontal plane). There have been few studies of visual discrimination of elevation, and fewer comparable studies of non-visual heading discrimination. To investigate human ability to estimate heading under more general conditions, we measured heading discrimination thresholds for azimuth and elevation in visual-only and vestibular-only conditions with observers oriented upright and side-down relative to gravity. Subjects were asked to discriminate heading azimuth or elevation relative to straight-ahead in a two-interval-forced-choice task. Experiments were conducted on a 6DOF Moog motion base with attached 90 × 90 deg visual display. The movement was a 1 sec Gaussian velocity profile with peak velocity and acceleration of 0.3 m/s and 1.13 m/s2, respectively. The visual scene depicted a 3D cloud of frontoparallel triangles rendered in stereo using shutter glasses. Visual thresholds were significantly lower than vestibular, and upright thresholds were generally lower than side-down. Visual-only results revealed that upright observers are equally sensitive to heading azimuth and elevation, but that side-down observers are better at discriminating head-centric azimuth (elevation relative to gravity). Vestibular-only results revealed that observers are better at discriminating head-centric azimuth, regardless of body orientation. In other words, vestibular sensitivity to heading depends more upon the direction of acceleration relative to the head, than relative to gravity. Gravitational and inertial accelerations are physically equivalent, so this result suggests that otolith sensitivity may not follow Weber's Law.
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