16 Οκτωβρίου 2013

Absence of adaptation effects in anisocoric monovision used in the correction of presbyopia

Almost all people over the age of 45 experience problems with near vision, due to the age-dependent loss of the eye’s ability to actively change focus. One solution that has been proposed is to increase the depth-of-focus of one eye by placing a small aperture in front of the natural pupil. This has the effect of increasing that eye’s passive depth-of-focus, allowing near objects to be seen more clearly. The other eye is left with a natural pupil, since the small pupil involves the some restriction in the visual field and a marked reduction in the light reaching the retina, particularly under dim conditions of illumination.


The unequal pupil diameters (anisocoria) and retinal illuminances implicit in this small-aperture monovision, however, lead to the interocular differences in the latency of Visual Evoked Potentials (VEP) and the Pulfrich effect (PE) in which the paths of moving objects may appear distorted, leading to possible hazard e.g. when driving.

Earlier work suggests that, when the PE is induced by placing a neutral density (ND) filter over one eye, partial adaptation occurs, reducing the PE by a factor of about 3 over a one-week period. If a similar adaptation occurred in small-aperture monovision this would be helpful in reducing any possible associated hazard.

Our studies (see below) provide solid evidence that, when presbyopia is corrected by the use of a monocular artificial pupil, in terms of a corneal inlay or a contact lens, the anisocoria produces marked interocular differences in visual latency (as measured with VEPs) and a Pulfrich effect. More recently we showed that these effects are not reduced by over a 7-day period of anisocoric vision, probably because the relationship between the two retinal illuminances is changing continually as the ambient illumination and diameter of the natural pupil change, making adaptation more difficult than in the case where the Pulfrich effect is induced with a monocular ND filter, when there is a constant ratio between the two retinal iilluminances.

Thus concerns remain over the safety of small-aperture monovision as an aid for presbyopes.

For more information about this work see the following studies:


Small-Aperture Monovision and the Pulfrich Experience: Absence of Neural Adaptation Effects

Reduced-aperture monovision for presbyopia and the Pulfrich effect

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