Perception of the dynamic visual vertical during sinusoidal linear motion.

The vestibular system provides information for spatial orientation. However, this information is ambiguous: because the otoliths sense the gravito-inertial force, they cannot distinguish gravitational and inertial components. As a consequence, prolonged linear acceleration of the head can be interpreted as tilt, referred to as the somatogravic effect. Previous modeling work suggests that the brain disambiguates the otolith signal according to the rules of Bayesian inference, combining noisy canal cues with the a priori assumption that prolonged linear accelerations are unlikely. Within this modeling framework the noise of the vestibular signals affects the dynamic characteristics of the tilt percept during linear whole-body motion. To test this prediction, we devised a novel paradigm to psychometrically characterize the dynamic visual vertical - as a proxy for the tilt percept - during passive sinusoidal linear motion along the inter-aural axis (0.33Hz motion frequency, 1.75m/s2 peak acceleration, 80cm displacement). While subjects (n=10) kept fixation on a central body-fixed light, a line was briefly flashed (5ms) at different phases of the motion, the orientation of which had to be judged relative to gravity. Consistent with the model's prediction, subjects showed a phase-dependent modulation of the dynamic visual vertical, with a subject-specific phase-shift with respect to the imposed acceleration signal. The magnitude of this modulation was smaller than predicted, suggesting a contribution of non-vestibular signals to the dynamic visual vertical. Despite their dampening effect, our findings may point to a link between the noise components in the vestibular system and the characteristics of dynamic visual vertical.

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