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1Department of Biophysics, 2Nijmegen Institute for Cognition and Information, and 3F. C. Donders Centre for Cognitive Neuroimaging, Radboud University Nijmegen, Nijmegen, The Netherlands
Submitted 18 December 2006; accepted in final form 20 February 2007
During prolonged rotation about a tilted yaw axis, often referred to as off-vertical axis rotation (OVAR), a percept of being translated along a conical path slowly emerges as the sense of rotation subsides. Recently, we found that these perceptual changes are consistent with a canalotolith interaction model that attributes the illusory translation percept to improper interpretation of the ambiguous otolith signals. The model further predicts that the illusory translation percept must be accompanied by slowly worsening tilt underestimates. Here, we tested this prediction in six subjects by measuring the time course of the subjective visual vertical (SVV) during OVAR stimulation at three different tilt-rotation speed combinations, in complete darkness. Throughout the 2-min run, at each left-ear-down and right-ear-down position, the subject indicated whether a briefly flashed line deviated clockwise or counterclockwise from vertical to determine the SVV with an adaptive staircase procedure. Typically, SVV errors indicating tilt underestimation were already present at rotation onset and then increased exponentially to an asymptotic value, reached at about 60 s after rotation onset. The initial error in the SVV was highly correlated to the response error in a static tilt control experiment. The subsequent increase in error depended on both rotation speed and OVAR tilt angle, in a manner predicted by the canalotolith interaction model. We conclude that verticality misjudgments during OVAR reflect a dynamic component linked to canalotolith interaction, superimposed on a tilt-related component that is also expressed under stationary conditions.
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