Interocular differences in visual latency induced by reduced-aperture monovision.

Purpose To explore the interocular differences in the temporal responses of the eyes induced by the monocular use of small-aperture optics designed to aid presbyopes by increasing their depth-of-focus. Methods Monocular and binocular pattern-reversal visual evoked potentials (VEPs) were measured at a mean photopic field luminance of 30cd/m2 in seven normal subjects with either natural pupils or when the non-dominant eye wore a small-aperture contact lens (aperture diameter 1.5, 2.5 or 3.5mm, or an annular opaque stop of inner and outer diameters 1.5 and 4.0mm respectively). Responses were also measured with varying stimulus luminance (5, 13.9, 27.2 and 45cd/m2) and a fixed 3.0mm artificial pupil. Results Mean natural pupil diameters were 4.7 and 4.4mm under monocular and binocular conditions respectively. The small-aperture contact lenses reduced the amplitude of the P100 component of the VEP and increased its latency. Inter-ocular differences in latency rose to about 2025ms when the pupil diameter of the non-dominant eye was reduced to 1.5mm. The measurements with fixed pupil and varying luminance suggested that the observed effects were explicable in terms of the changes in retinal illuminance produced by the restrictions in pupil area. Conclusions The anisocoria induced by small-aperture approaches to aid presbyopes produces marked interocular differences in visual latency. The literature of the Pulfrich effect suggests that such differences can lead to distortions in the perception of relative movement and, in some cases, to possible hazard.