Does the presbyopic visual system adapt to contact lenses?
We studied whether task performance improves during the initial wearing period of selected presbyopic contact lens corrections. In three previous studies significant decreases in performance on three selected tasks were measured before and after initial wearing periods to monovision contact lenses (N = 18, 8 weeks of wear), concentric bifocal lenses (N = 32, 8 weeks), and diffractive and monovision lenses (N = 20, 3 weeks). In each study, the control condition was distance contact lenses with reading spectacles. The decreases in performance speed ranged from 2 to 15% compared to control. In this study, we retrospectively analyze the data to determine whether the task performance decreases relative to control were significantly different after the initial wearing period than at the time of dispensing--i.e., did measures of task performance show an adaptation to the presbyopic contact lens correction? Performance adaptation on each task and for each study was analyzed statistically using the Wilcoxon matched pair signed rank test of each subject's mean performance differences (test vs. control condition) at dispensing compared to the performance at the end of the wearing period. There was no improvement in task performance with monovision or diffractive bifocal lenses. However, we found significant improvements in task performance with concentric bifocal lenses (p < 0.05). Measures of visual acuity and stereoacuity did not improve with these lenses during the studied wearing period. Thus, these basic visual skills did not adapt. The ability of patients to learn how to use reduced visual acuity to perform complex tasks is a possible explanation for the improved task performance with concentric bifocal lenses compared to control.