Dynamic visual acuity with telescopic spectacles: improvement with adaptation.

Telescopic spectacles are used as aids for the visually impaired in order to increase effective visual acuity. Because ocular stabilization reflexes are not fully compensatory when telescopic spectacles are worn, head motion would be expected to produce retinal image motion which could decrease visual acuity. Using 1.0 Hz sinusoids of vertical axis head rotation, we investigated the effect of head velocity and telescopic spectacle magnification on binocular dynamic visual acuity (DVA), the acuity during head motion, in 34 normally sighted subjects. The visual field peripheral to the telescopes was masked. Up to a head velocity amplitude of 30 degrees/sec, DVA was insensitive to head velocity for X2 telescopic spectacles. For X4 and, to a greater degree, X6 telescopic spectacles, DVA decreased progressively as head velocity increased. DVA measurements were repeated after a 15 min adaptation period, during which a distant video monitor was viewed using telescopic spectacles. For X4 telescopic spectacles, DVA increased significantly after adaptation. With an unobstructed peripheral visual field, initial DVA with X4 telescopic spectacles was equal to adapted DVA with peripheral vision occluded, but adaptation produced no further improvement in DVA with the peripheral field unobstructed. These data indicate that the visual acuity obtained with telescopic spectacles is substantially reduced under conditions where head motion occurs, potentially reducing the functional value of these devices in low vision rehabilitation. The adverse effect of head motion on DVA may be reduced by adaptation.