Abstract

Telescopic spectacles are potentially advantageous as aids for visually impaired patients, who might be expected to uniformly benefit from optical magnification. However, these relatively costly devices fail to benefit a substantial number of patients with low vision¹. Based upon retrospective¹ and prospective clinical data², it has been hypothesized that a major cause of rehabilitation failure with telescopic spectacles is retinal image instability during involuntary head motion. This retinal slip hypothesis supposes that ubiquitous head motion, optically magnified by telescopic spectacles, overwhelms compensatory ocular motor reflexes to produce slipping motion of images on the retina, which degrades dynamic visual acuity (DVA) sufficiently to negate the magnification advantage of the telescopes. Several lines of evidence support this hypothesis. The visual-vestibulo-ocular reflex (VVOR) is the principal compensatory mechanism stabilizing the retina during head movements. The VVOR gain of both normally sighted³ and low vision subjects¹ fails to increase sufficiently to match the magnification of telescopic spectacles, a situation presumed to result in retinal image motion during head motion. In normally sighted subjects, actual retinal image motion has been measured during DVA tasks conducted both with vertically moving optotypes, and during vertically imposed head motion⁴. That study found that, regardless of whether retinal image motion was produced by moving optotypes or moving head, DVA in normal subjects was independent of image velocity up to 2°/sec, but declined with the 0.6 power of image velocity for greater slip. The present investigation was conducted to determine if patients with low vision have a similar relationship between DVA and retinal image slip velocity.

© 1995 Optical Society of America