Many low vision patients have impaired central acuity but cannot successfully use telescopic spectacles despite the theoretical benefit from magnification. This may be because visual rehabilition with telescopic spectacles is limited by instability of the magnified retinal image during involuntary head movements, and during coordinated eye-head movements necessitated by visual search. Retinal image stability is ordinarily provided by the visual-vestibulo- ocular reflex (VVOR), produced by interaction of the vestibulo-ocular reflex with visual and predictive mechanisms (visual vestibular interaction VVI). VVOR performance with telescopic spectacles must be quantitatively appropriate to magnification to avoid substantial impairment of dynamic visual acuity during head movements. The overall aim is to quantitatively evaluate mechanisms of visual-vestibular interaction in the vertical plan, as determinants of useful vision with telescopic spectacles during physiologic head movements. Eye and head movements will be measured using magnetic search coils, supplemented by infrared oculography. The effect of telescopic spectacles on vertical VVOR will be quantitatively characterized in the physiologic range of active and passive head velocities and frequencies, in normally sighted and low vision adults. Predictable and unpredictable head movements will be studied with aid of mathematical modelling and simulation in order to ascertain the role of predictive, motor efference copy, and other ocular motor mechanisms in supplementing VVI. Findings will be correlated with separate measurements of visually guided tracking. The relationship between instantaneous vertical retinal image slip velocity, and dynamic visual acuity, will be studied in relation to viewing strategies employed with telescopic spectacles. By studying horizontal and vertical eye and head movements during unrestrained visual search, experiments will identify characteristics of optimal visual search strategies with telescopic spectacles. Modification of search strategy to minimize retinal image instability and search time will also be evaluated. These data will enhance basic understanding of human vertical eye movements, and provide clinical insight into an important problem in clinical low vision.
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