Higher-order cognitive processes moderate body tilt effects in vection. Guterman, P. & Allison, R. S. Displays, 58:44-55, 2019.
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Changing head orientation with respect to gravity changes the dynamic sensitivity of the otoliths to linear accelerations (gravitational and inertial). We explored whether varying head orientation and optic flow direction relative to gravity affects the perception of visually induced self-motion (vection) in two experiments. We confirmed that vertical optic flow produces stronger vection than horizontal optic flow in upright observers. We hypothesized that if this was due to aligning the simulated self-motion with gravity, then interaural (as opposed to spinal) axis motion while lying on the side would provide a similar vection advantage. Alternatively, motion along the spinal axis could enhance vection regardless of head orientation relative to gravity. Finally, we hypothesized that observer expectation and experience with upright locomotion would favour horizontal vection, especially when in upright posture. In the first experiment, observers stood and lay supine, prone, left and right side down, while viewing a translating random dot pattern that simulated observer motion along the spinal or interaural axis. Vection magnitude estimates, onset, and duration were recorded. Aligning the optic flow direction with gravity enhanced vection in side-laying observers as reflected by either a bias for interaural rather than spinal flow or by an elimination/reduction of the spinal advantage compared to upright. However, when overlapping these signals was not possible—as in the supine and prone posture—spinal axis motion enhanced vection. Furthermore, perceived scene structure varied with head orientation (e.g., dots were seen as floating bubbles in some conditions). To examine the influence of scene structure, in the second experiment we compared vection during simulated motion with respect to two environments: a rigid pipe structure that looked like a complex arrangement of plumbing pipes, and a field of dots. Interestingly, vertical optic flow with the pipes stimulus produced a similar experience to that of riding an elevator and tended to enhance vection. Overall, we found that vection depended on the direction of both the head orientation and visual motion relative to gravity, but was also influenced by the perceived scene context. These findings suggest that, in addition to head tilt relative to gravity, that higher-order cognitive processes play a key part in the perception of self-motion.

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