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This thesis describes the results of a research project to investigate vestibular as- pects of visual updating in roll-tilted subjects. The first part (Chapter 2 and 3) fo- cuses on errors in verticality perception of tilted subjects. We tested whether the SVV was influenced by semicircular-canal signals. Furthermore, we tried to quan- tify a bistability phenomenon at large tilts that has been reported earlier, but only in a qualitative sense. In the second part (Chapter 4 and 5) we investigated canal and otolith contributions to visual stability during dynamic head rotations. We de- termined the relative role of the canals and the otoliths in this mechanism, and also tried to gain more insight into the otolith-disambiguation process. The next sections summarize the main questions and conclusions of the previous chapters.

CHAPTER 2 Results of earlier spatial-orientation studies focusing on the sense

of verticality have emphasized an intriguing paradox. Despite evidence that nearly veridical signals for gravicentric head orientation and egocentric visual stimulus orientation are available, roll-tilted subjects err in the direction of the long body axis when adjusting a visual line to vertical in darkness (Aubert effect). This has led to the suggestion that a central egocentric bias signal with fixed strength and direction acts to pull the perceived vertical to the subjects’ zenith (M-model).

In the present study the subjective visual vertical (SVV) was tested in six human subjects, across the entire 360◦ range. For comparison, body-tilt estimates from four subjects where collected in a separate series of experiments. For absolute tilts up to about 135◦SVV responses showed a gradually increasing Aubert effect which could not be attributed to errors in perceived body tilt but was nicely in line with the M-model. At larger absolute tilts, SVV errors abruptly reversed sign, now showing a pattern concordant with errors in body-tilt estimates but incompatible with the M- model. These results suggest that, in the normal working range, the perception of external space and the perception of body posture are based on different processing of body-tilt signals. Beyond this range, both spatial-orientation tasks seem to rely mainly on a common tilt signal.

CHAPTER 3 A striking feature of visual verticality estimates in the dark is under-