Visual fatigue symptoms are associated with a reduction in accommodative performance. The study aimed to understand the impact of near viewing on visual fatigue and the accommodative system. A custom-made program was created for the free-space facility test to objectively measure dynamic changes of the accommodative accuracy and stability and obtain response accuracy and reaction time.
Real-time measurements of accommodation were obtained with an open-field autorefractor from 23 young adults while reading and performing a free-space facility test (targets at 500 and 25 cm) using a computer program which measured subjects' Reaction Time (RT), response accuracy, and accommodative accuracy and stability while maintaining focus at the targets for 8 seconds. In a crossover design, each subject performed a fatigue-inducing task (digital gaming) for 25 minutes in three viewing conditions tested on three different days: near-binocular (NB), far-binocular (FB), near-monocular (NM). Pairwise comparison was conducted to compare visual discomfort rating, RT, response accuracy, accommodative, and pupillary measurements before and after gaming. Mixed-Model ANCOVA was used to compare the effects of the fatigue-inducing task (Conditions), symptomatic vs. asymptomatic group (Group), post-task order (Order), and their interactions on all target measurements with the pre-test measurement as covariate.
After the fatigue-inducing task, subjective discomfort rating on Visual Discomfort Survey (VDS) increased significantly (P = P P= .001), and facility at near (P = .001). The overall score of VDS was significantly different between conditions (P = 0.04) with higher discomfort rating after the NB condition. In the facility test, accommodative instability was higher after FB than after NB and NM with both far facility (P = .03 and .04, respectively) and near facility (P = .004 and .001, respectively) tasks. The symptomatic group showed higher pupillary (P = .002) and accommodative (P = .04) instability than the asymptomatic group at near facility. At near facility, pupil size and accommodation were more unstable under the 2nd post-task order (free-space facility then reading) than under the 1st post-task order (reading then free-space facility).
Increased VDS total score after the fatigue-inducing task partially supported our hypothesis that sustained near viewing of digital devices would cause visual discomfort though we cannot attribute the effect to the accommodative system alone. Consistent with previous findings, pupil diameter decreased significantly after the fatigue-inducing task in both tests (reading and facility); however, no accommodative changes were observed. It is possible that the effect of the fatigue-inducing task was not strong enough to change all oculomotor functions; still, the pupil constricted increasing the depth of focus, but without a measurable change in accommodative power. The finding that higher subjective discomfort was reported after NB than after NM suggests that the involvement of both near-focusing (accommodation) and binocular coordination (convergence) is more stressful than the accommodative system alone. Similarly, it’s more challenging for an anomalous oculomotor system to cope with visual stress, which may explain the finding of the higher pupillary and accommodative instability with the symptomatic group compared to the asymptomatic group, especially when the test occurred immediately after the fatigue-inducing task.
Even with brief near digital viewing, visual discomfort significantly increased as measured by VDS in all subjects. This impact was more evident in symotomatic subjects. The pupillary response seemed to readily respond when following the fatigue-inducing task. The effect of the visual stress was stronger among symptomatic subjects and was manifest clearly when measured immediately after the fatigue-inducing task. Interruption from other tasks may mitigate the effect of near-viewing stress to the oculomotor system and make it harder to diagnose. No accommodative changes were observed except the increment of accommodative instability among symptomatic subjects during the dynamic facility test. The free-space facility program, created for this study, allows accurate assessment of the subject’s response accuracy, RT, and accommodative dynamics (speed, accuracy, and stability of response) when shifting focusing distances in a naturalistic environment. Future studies should consider an increase in the fatigue-inducing level and duration which may elicit more changes in the accommodative system to aid in understanding accommodative behavior under visual stress. With a simple modification of the testing protocol (such as the fixation time), the program will allow for assessment of different components in the oculomotor models.
Visual fatigue, Accommodation, Free-space accommodative facility, Accommodation stability
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