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Paul R. MacNeilage, Daniel Berger, Martin S. Banks, Heinrich H. Buelthoff; Visual cues are used to interpret gravito-inertial force. Journal of Vision 2004;4(8):142. doi: 10.1167/4.8.142.
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Humans use visual and non-visual cues to estimate body orientation and self-motion relative to gravity. Non-visual cues include forces acting on the body, which are signaled by the vestibular and somatosensory systems. These cues are ambiguous indicators of the direction of gravitational force because of Einstein's equivalence principle: any linear accelerometer measures the sum of forces. Thus, forces due to gravity and to acceleration are confounded. Visual cues to body orientation and self-motion relative to gravity could resolve the ambiguity. Optic flow is the primary visual cue to self-motion. It could be used to estimate self-acceleration, and thereby estimate the component of the vestibular-somatosensory signal caused by acceleration as opposed to gravity. Additional visual cues to body orientation include environmental features that have a fixed orientation with respect to gravity, such as the horizon. Using a 6-df motion platform with a large visual display, we examined whether visual cues are used to disambiguate the vestibular-somatosensory signal. We presented different combinations of vestibular-somatosensory signals (by pitching the platform) and visual cues (acceleration specified by optic flow and orientation by horizon pitch) and asked observers to make judgments about perceived body orientation and perceived forward acceleration. They reported in which of two intervals they were more pitched and in which they were more accelerated. Vestibular-somatosensory and horizon pitch affected orientation and acceleration judgments. Optic flow affected acceleration judgments but not orientation judgments. We present a computational model of how cues may be combined to derive separate estimates of gravity and other inertial forces.
AFOSR Grant F49620, NIH Training Grant T32 EY07043-26, Deutsche Forschungsgesellschaft DFG, SFB 550
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