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1 Biophysics, Radboud University Nijmegen, Nijmegen, Netherlands; Nijmegen Institute for Cognition and Information, Radboud University Nijmegen, Nijmegen, Netherlands
2 Nijmegen Institute for Cognition and Information, Radboud University Nijmegen, Montessorilaan 3, Nijmegen, 6525 HR, Netherlands; FC Donders Centre for Cognitive Neuroimaging, Radboud University Nijmegen, Nijmegen, Netherlands
3 Biophysics, Radboud University Nijmegen, Nijmegen, Netherlands
* To whom correspondence should be addressed. E-mail: p.medendorp{at}nici.ru.nl.
To assess the effects of degrading canal cues for dynamic spatial orientation in human observers, we tested how judgments about visual-line orientation in space (subjective visual vertical task, SVV) and estimates of instantaneous body tilt (subjective body-tilt task, SBT) develop in the course of three cycles of constant-velocity roll rotation. These abilities were tested across the entire tilt range, in separate experiments. For comparison, we also obtained SVV data during static roll tilt. We found that, as tilt increased, dynamic SVV responses became strongly biased toward the head pole of the body axis (A-effect), as if body tilt was underestimated. However, upon entering the range of near-inverse tilts, SVV responses adopted a bimodal pattern, alternating between A-effects (biased toward head-pole) and E-effects (biased toward feet-pole). Apart from an onset effect, this tilt-dependent pattern of systematic SVV errors repeated itself in subsequent rotation cycles, with little sign of worsening performance. Static SVV responses were qualitatively similar and consistent with previous reports, but showed smaller A-effects. By contrast, dynamic SBT errors were small and unimodal, indicating that errors in visual-verticality estimates were not caused by errors in body-tilt estimation. We discuss these results in terms of predictions from a canal-otolith interaction model, extended with a leaky integrator and an egocentric bias mechanism (Vingerhoets et al. 2006; 2007). We conclude that the egocentric-bias mechanism becomes more manifest during constant velocity roll-rotation and that perceptual errors due to incorrect disambiguation of the otolith signal are small, despite the decay of canal signals
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