VISUAL-ACUITY FOR OPTOTYPES WITH IMAGE MOTIONS SIMULATING CONGENITAL NYSTAGMUS

1. In individuals with idiopathic congenital nystagmus (CN), visual acuity is typically found to be slightly to moderately reduced, which is attributed to the incessant to and fro motion of their eyes. Previous studies have identified several eye movement parameters that are related to visual acuity. These parameters include the intensity of the nystagmus (i.e. amplitude x frequency), the duration of foveation periods (i.e. intervals of relatively low eye velocity when the image of a target is on or near the fovea), and the variability of eye position across successive foveations. 2. The above parameters are interrelated in persons with CN, making their individual effects on visual acuity difficult to ascertain. Therefore, we measured visual acuity in normal subjects with retinal image motions simulating those in CN. Subjects monocularly viewed acuity targets in a mirror that oscillated with either a sinusoidal (to mimic pendular nystagmus) or a ramp waveform (to mimic jerk nystagmus). The intensity of image motion was varied systematically for both waveforms, as was the amount of beat-to-beat position variability in the low-velocity portions of the simulated waveforms (i.e. the simulated foveation periods). The duration of the simulated foveation periods was also varied, for the ramp waveform. 3. Visual acuity worsened as the duration of simulated foveation periods was reduced and when position variability was added to the waveforms. Although acuity was unaffected by changes in the intensity of imposed ramp motion, it worsened when the intensity of imposed sinusoidal motion was increased, presumably because the duration of simulated foveations was concomitantly reduced. With imposed ramp motion of the target, optimal acuity was attained when the duration of simulated foveation periods reached about 60 ms. 4. Under conditions of comparable retinal image motion, acuities are poorer in the majority of individuals with idiopathic CN than in normal subjects. This suggests that acuity may be limited by sensory deficits that are induced during early life, rather than by retinal image motion per se, in many patients with CN.