Influence of panoramic cues during prolonged roll-tilt adaptation on the percept of vertical.

The percept of vertical, which mainly relies on vestibular and visual cues, is known to be affected after sustained whole-body roll tilt, mostly at roll positions adjacent to the position of adaptation. Here we ask whether the viewing of panoramic visual cues during the adaptation further influences the percept of the visual vertical. Participants were rotated in the frontal plane to a 90° clockwise tilt position, which was maintained for 4-minutes. During this period, the subject was either kept in darkness, or viewed panoramic pictures that were either veridical (aligned with gravity) or oriented along the body longitudinal axis. Errors of the subsequent subjective visual vertical (SVV), measured at various tilt angles, showed that the adaptation effect of panoramic cues is local, i.e. for a narrow range of tilts in the direction of the adaptation angle. This distortion was found irrespective of the orientation of the panoramic cues. We conclude that sustained exposure to panoramic and vestibular cues does not adapt the subsequent percept of vertical to the direction of the panoramic cue. Rather, our results suggest that sustained panoramic cues affect the SVV by an indirect effect on head orientation, with a 90° periodicity, that interacts with a vestibular cue to determine the percept of vertical.

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 gravitoinertial 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 interaural axis (0.33 Hz motion frequency, 1.75 m/s2 peak acceleration, 80 cm displacement). While subjects (n=10) kept fixation on a central body-fixed light, a line was briefly flashed (5 ms) 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 nonvestibular 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.NEW & NOTEWORTHY A fundamental question in neuroscience is how the brain processes vestibular signals to infer the orientation of the body and objects in space. We show that, under sinusoidal linear motion, systematic error patterns appear in the disambiguation of linear acceleration and spatial orientation. We discuss the dynamics of these illusory percepts in terms of a dynamic Bayesian model that combines uncertainty in the vestibular signals with priors based on the natural statistics of head motion.