A Common Prescription Lens Correction Causes Motion Illusions in the Presbyopic and General Populations

Monovision is a common correction for presbyopia that focuses one eye at far distances and the other at near distances, resulting in a difference in blur between the eyes. Because blur increases the speed of visual processing by a few milliseconds, these optical conditions can induce dramatic misperceptions of the distances and three-dimensional directions of moving objects. To date, the illusion has been demonstrated only in individuals without presbyopia.

We analyze both the induced processing speed differences and the visibility of the resulting illusions in presbyopic (n = 17, 22.2 ± 5.0 years) and non-presbyopic (n = 36, 54.4 ± 5.9 years) populations, with proportions approximately matching those in the general population. Participants viewed two horizontally moving strips of bars on an autostereoscopic display with interocular blur and light-level differences, and reported which strip appeared closer in depth. Interocular delay and an illusion visibility index-the ratio of interocular delay and the detection threshold-were computed for each participant in each condition.

Delays were highly consistent in both the presbyopic and general populations. Interocular differences in optical blur and light-level caused highly consistent interocular differences in processing speed in all participants, although they created visible (suprathreshold) illusions in only a subset of participants. This subset, however, included individuals with processing delays that were many times larger than the detection threshold. Such individuals are likely to be afflicted by large, highly visible illusions in real-world conditions.

Methods for identifying high-risk individuals and for reducing or eliminating the illusions are discussed.