Verticality Perception During Off-Vertical Axis Rotation

1 Department of Biophysics, 2 Nijmegen Institute for Cognition and Information, and 3 F. 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 canal–otolith 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 canal–otolith interaction model. We conclude that verticality misjudgments during OVAR reflect a dynamic component linked to canal–otolith interaction, superimposed on a tilt-related component that is also expressed under stationary conditions. Address for correspondence: R.A.A. Vingerhoets, Department of Biophysics, Radboud University Nijmegen, PO Box 9101, 6500 HB Nijmegen, The Netherlands (E-mail: r.vingerhoets{at}science.ru.nl )