Abstract

One obvious limitation of telescopic spectacles (TS) is that they provide magnification at the expense of visual field diameter. Thus, visually impaired people who use TS as visual aids must make head movements to locate targets of interest for scrutiny. When retinal image velocity exceeds 2-3°/sec, visual acuity declines as a log-linear function of this slip velocity¹. During unmagnified viewing, the visually-enhanced vestibulo-ocular reflex (VVOR) produces eye movements that compensate for head movements to prevent instability of retinal images of fixed objects. The gain of the VVOR, defined to be the ratio of compensatory eye velocity to head velocity, normally has an ideal value of 1.0 for distant targets. Unfortunately, with TS a simple strategy of making rapid head movements does not necessarily facilitate the efficiency of visual search, since these devices destabilize retinal images by magnifying the relative velocity of the head movements. Ideal VVOR gain with TS would be equal to TS magnification. However, data obtained during passively-imposed and self-generated head rotations suggests that augmentation of VVOR gain by magnified vision is insufficient to attain ideal performance². Thus, when TS are worn, rapid head movements could degrade visual acuity and lead to target misses.

© 1996 Optical Society of America