Contact lenses can cause the reverse Pulfrich effect and anti-Pulfrich monovision corrections can eliminate it

Abstract

Interocular differences in image blur can cause dramatic misperceptions of the distance and three-dimensional direction of moving objects. This new illusion--the reverse Pulfrich effect--is caused by the optical conditions induced by monovision, a common correction for presbyopia. Fortunately, anti-Pulfrich monovision corrections, in which the blurring lens is slightly darkened, can eliminate the illusion for a wide range of viewing conditions. However, the reverse Pulfrich effect and the efficacy of anti-Pulfrich corrections have previously been demonstrated only with trial lenses. This situation should be addressed, for both clinical and scientific reasons. First, monovision is most commonly prescribed with contact lenses. It is important to replicate these effects in the most common monovision delivery system. Second, trial lenses of different powers, unlike contacts, cause large magnification differences between the eyes. To confidently attribute the reverse Pulfrich effect to differences in optical blur between the eyes, and to ensure that the reported effect sizes are reliable, one must control for magnification. Here, in a within observer study with five separate experiments, we demonstrate i) that contact lenses induce reverse Pulfrich effects that are indistinguishable from those induced by trial lenses, ii) that overall magnification differences do not cause or impact the Pulfrich effect, and iii) that anti-Pulfrich corrections (i.e. darkening the blurring lens) are equally effective when induced by contact lenses and by trial lenses.