Abstract
Purpose: :
Traditionally presbyopia is treated using corrective bifocal or multifocal lenses. An alternative is to correct one eye for near and the other for distance with a method known as "monovision". It is known that differential interocular blur can degrade stereoacuity, and recent studies have confirmed that monovision treatment increases stereoacuity thresholds. However, stereoacuity tests do not assess disparity sensitivity in the coarse range. Given the proven link between stereopsis and stability, we have measured the short-term effects of induced monovision on stereopsis over a broad range of fine (fused) and coarse (diplopic) disparities at both near and far viewing distances.
Methods: :
Stimuli were presented dichoptically using a time-sequential polarized stereoscopic display. During each trial a line was presented for 300 ms with either crossed or uncrossed disparity above a zero disparity fixation cross. Participants indicated the direction of the depth offset. In one session baseline performance was assessed with optimal optical correction. In another, monovision was induced by adding -1D and +1D lenses in front of the dominant and non-dominant eyes respectively. We assessed performance at distances of 62 and 300 cm in counterbalanced blocks. Within each block, the stimuli were presented at 5 fine disparities ranging from 60 to 2400 arcsec and 5 coarse disparities ranging from 1o to 3.5o.
Results: :
Induced monovision resulted in decreased accuracy relative to baseline in the fine disparity range, but effects were minimal in the coarse range. Monovision also had a larger impact on performance at a viewing distance of 300 cm than at 62 cm.
Conclusions: :
Induced monovision not only increases stereoacuity thresholds, but degrades depth discrimination across the range of fusable disparities in young observers. This effect on fine disparity is accentuated at larger viewing distances typical of fixation distances during walking, suggesting that stability during locomotion may be degraded. However, we also found that coarse stereopsis was relatively spared, and this may offset the observed losses.
Keywords: binocular vision/stereopsis • presbyopia