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* To whom correspondence should be addressed. E-mail: a.bronstein{at}imperial.ac.uk.
The subjective visual vertical (SVV) is usually considered a measure of otolith function. Herewith we investigate the influence of semicircular canal stimulation (SCC) on the SVV by rotating normal subjects in yaw about an earth-vertical axis, with velocity steps of +/- 90°/s, for 60s. SVV was assessed by setting an illuminated line to perceived earth vertical in darkness, during a per- and post-rotary period. Four head positions were tested: upright, 30° backwards (chin up) or forwards, and approximately 40° forwards from upright. During head upright/backwards conditions a significant SVV tilt (p<0.01) in the direction opposite to rotation was found, which reversed during post-rotary responses. The rotationally induced SVV tilt had a time constant of decay of approximately 30s. Rotation with the head 30° forwards did not affect SVV while the 40° forwards tilt caused a direction reversal of SVV responses, compared to head upright/backwards. 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. 3-D video-oculography showed that SVV and torsional eye position measurements were a) highly cross-correlated (0.83) and b) in the direction opposite to the slow phase torsional vestibulo-ocular 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. 5) Clinically, abnormal SVV tilts cannot be considered a specific indication of otolith system dysfunction.
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