Telescopic spectacles, highly magnifying visual aids mounted in spectacle frames, markedly alter the visual consequences of head movements. To evaluate the effect of this altered visual feedback on head stability, angular head velocity of normally sighted and low vision subjects was measured in the roll, pitch, and yaw axes. Measurements were made under two postural conditions: (1) quiet standing; and (2) walking in place, as well as three visual conditions: (1) eyes closed; (2) unmagnified vision; and (3) vision with 4× binocular telescopic spectacles. For normal subjects during quiet standing, both unmagnified vision and vision with telescopic spectacles tended to reduce spontaneous head velocity in all axes as compared to the eyes-closed condition. However, in low vision subjects neither unmagnified vision nor vision with telescopic spectacles produced significant changes in values of head velocity relative to those measured with eyes closed. Spontaneous head velocities for standing low vision subjects tended to be higher than in normal subjects, although not all differences were statistically significant. During walking in place, Fourier analysis demonstrated prominent frequency components related to harmonics of the walking frequency under all viewing conditions. In normal subjects, vision with telescopic spectacles, to a greater degree than unmagnified vision, reduced head velocity during walking in the roll and yaw, but not the pitch, axes. For low vision subjects, significant reductions in head velocity during walking were observed only during vision with telescopic spectacles. These findings indicate that vision reduces angular instability of the head during standing and walking. Magnification produced by telescopic spectacles further improves head stability under some conditions, although the effect of vision is least evident in the pitch axis. The stabilizing effect of vision is reduced in low vision subjects.
Telescopic spectacles are commonly prescribed to the visually impaired. However, many such low vision patients cannot use spectacle magnifiers effectively. This may be because these patients cannot maintain adequate stability of the magnified image on the retina during head movements. Head movements are the ubiquitous consequence of ambulation, tremor, and postural instability.
Telescopic spectacles are frequently prescribed for visually impaired patients, but clinical observation reveals that low vision patients often do not achieve expected functional use of these costly visual aids. This may be a result of patients’ inability to maintain adequate stability of magnified images on the retina during unintended head movements caused by ambulation, tremor, and postural instability. Although the visual-vestibulo-ocular reflex (VVOR) produces eye movements that partially compensate for head movements, this compensation is incomplete when images are magnified by head-mounted telescopes.1 It has been demonstrated in normally sighted subjects wearing telescopic spectacles that head motion significantly decreases dynamic visual acuity (DVA), the acuity during head motion.2 We hypothesized that insufficient compensation of head instability might cause reduced DVA and motion sickness, and might thus predict failure of low vision patients to make functional use of telescopic spectacles. We have been testing this retinal image instability hypothesis to develop methods that may be useful in prediction of the successful use of telescopic spectacles.
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