Abstract
This thesis describes the results of a research project to investigate vestibular aspects of visual updating in roll-tilted subjects. The focus of the first part is on errors in verticality perception of tilted subjects. Although tilted subjects accurately known how much they are tilted and also know the line orientation relative to their body, they are not able to estimate the line orientation relative to gravity. An important model that tries to explain this relies solely on otolith signals, but other studies suggests that canal signals play a role. Our results confirm that the canals do not play a role at tilt up to about 135 degrees. However, at large tilts a sudden transition to errors of opposite sign occurs. After the transition, we find canal effects and also a strong correlation between verticality perception and body-tilt perception. Our study is the first to thoroughly quantify and discuss these phenomena. The second part of this thesis focuses on another question: how are signals of the otoliths and semicircular canals combined to update the internal representation of visual space? We tested visual stability during dynamic head rotations at different frequencies, in conditions with and without otolith cues. Our results show that subjects severely underestimate head rotation. An illusory translation percept is present at higher frequencies. The results shows that the canal contribution to visual orientation perception has high-pass characteristics. The otolith contribution is low-pass and relatively small. Current models cannot explain these findings. We suggest that the brain forms two estimates of head tilt, one based on otolith signals and one based on canal signals, and that the final estimate is a weighted sum of these two. The violations of visual position constancy are compatible with leaky integration of an internal estimate of head acceleration.
