GAZE STABILIZATION DURING DYNAMIC POSTUROGRAPHY IN NORMAL AND VESTIBULOPATHIC HUMANS

Dynamic posturography by measurement of center of pressure (COP) is a widely employed technique for evaluating the vestibular system. Howeve r, the relationship of COP motion to vestibule-ocular reflex (VOR) fun ction and image stability on the retina has not been determined previo usly. To assess these relationships, we report gaze, head, and trunk s tability during dynamic posturography in 11 normal volunteers, 7 subje cts with unilateral vestibular lesions, and 3 subjects with bilateral vestibular lesions. Posturographic tasks consisted of standing still a nd standing on a platform that was sliding (0.2 Hz), tilting (0.1 Hz), or covered with a foam cushion 6 cm thick while tilting (0.1 Hz). Eac h perturbation was imposed in the anterior-posterior and repeated in t he medial-lateral direction, in both Light and darkness. Subjects view ed (or in darkness remembered) a target located 50, 100, or 500 cm dis tant. COP, angular eye position, and angular and linear orbit and trun k positions were measured using magnetic search coils and flux gate ma gnetometer sensors. With the target visible, the velocity of image mot ion on the retina was on average always less than 1 degrees/s, well wi thin the range consistent with high visual acuity. In darkness, gaze v elocity increased for normal and vestibulopathic subjects. During tilt , vestibulopathic subjects had a significantly greater gaze velocity t han controls. Gain of the angular VOR (eye velocity/head velocity) was significantly lower in darkness than in Light and in vestibulopathic as compared to control subjects. Gain of the VOR was significantly cor related with gaze instability, but variation in VOR gain accounted for only 20-40% of the variance. In darkness, the velocity of the COP was significantly greater in vestibulopathic than control subjects for ev ery condition tested. In light, this difference was small and often no t significant. Although spectral analysis of the COP indicated frequen cies above 1 Kz that were not observed in motion of the trunk and orbi t, root mean square (RMS) velocities of the trunk and orbit in the hor izontal plane were higher in darkness and in vestibulopathic subjects, mirroring COP findings. Only in vestibulopathic subjects tested in da rkness was there a correlation between COP velocity and gaze velocity; COP velocity was otherwise uncorrelated with gaze. Gaze velocity was greater with near than with distant targets. Vertical VOR gain was hig her with near targets. No other significant effects of target distance were found. Head movement strategy, VOR gain, and COP were all unaffe cted by target proximity. These data show that gaze velocity measureme nts during dynamic posturography in darkness are sensitive to vestibul ar loss. With a visible target, both COP and gaze stability of vestibu lopathic subjects are difficult to distinguish from normal. During vis ual feedback, it is likely that image stabilization over the range of frequencies tested is achieved through better head stability and throu gh visual tracking, allowing vestibulopathic subjects to maintain adeq uate visual acuity.