Effect of Semicircular Canal Stimulation on the Perception of the Visual Vertical

Academic Department of Neuro-Otology, Division of Neuroscience and Psychological Medicine, Faculty of Medicine, Imperial College London, Charing Cross Hospital, London W6 8RF, United Kingdom Submitted 24 October 2002; accepted in final form 15 March 2003 The subjective visual vertical (SVV) is usually considered a measure of otolith function. Herewith we investigate the influence of semicircular canal (SCC) stimulation on the SVV by rotating normal subjects in yaw about an earth-vertical axis, with velocity steps of ± 90°/s, for 60 s. SVV was assessed by setting an illuminated line to perceived earth vertical in darkness, during a per- and postrotary period. Four head positions were tested: upright, 30° backward (chin up) or forward, and 40° forward from upright. During head upright/backward conditions, a significant SVV tilt ( P < 0.01) in the direction opposite to rotation was found that reversed during postrotary responses. The rotationally induced SVV tilt had a time constant of decay of 30 s. Rotation with the head 30° forward did not affect SVV, whereas the 40° forward tilt caused a direction reversal of SVV responses compared with head upright/backward. Spearman correlation values (Rho) between individual SCC efficiencies in different head positions and mean SVV tilts were 0.79 for posterior, 0.34 for anterior, and – 0.80 for horizontal SCCs. Three-dimensional video-oculography showed that SVV and torsional eye position measurements were highly correlated (0.83) and in the direction opposite to the slow phase torsional vestibuloocular reflex. In conclusion: 1 ) during yaw axis rotation without reorientation of the head with respect to gravity, the SVV is influenced by SCC stimulation; 2 ) this effect is mediated by the vertical SCCs, particularly the posterior SCCs; 3 ) rotationally induced SVV changes are due to torsional ocular tilt; 4 ) SVV and ocular tilts occur in the "anticompensatory," fast phase direction of the torsional nystagmus; and 5 ) clinically, abnormal SVV tilts cannot be considered a specific indication of otolith system dysfunction. Address for reprint requests: A. M. Bronstein, Academic Dept. of Neurootology, Div. of Neuroscience and Psychological Medicine, Imperial College London, Charing Cross Hospital, Fulham Palace Road, London W6 8RF UK (E-mail: a.bronstein{at}imperial.ac.uk ).