Abstract
Purpose :
Fusional vergence ranges are typically measured subjectively using Risley prisms or a prism bar. In this study, new methods to measure fusional vergence ranges at near objectively were validated against the two conventional tests used in clinics.
Methods :
A total of 46 typical adults (23.3±3.0 years) participated in the study. Participants’ BO (base out) and BI (base in) near fusional vergence ranges were measured objectively in an haploscopic set-up and recording eye movements with an Eyelink 1000 Plus (SR Research) at 500Hz. Firstly, stimulus disparity changed smoothly at 1PD/s up to 45PD for both divergence (BI) and convergence (BO), mimicking a Risley prism. Secondly, disparity was changed in steps of 2PD every 2 seconds mimicking a prism bar. Break and recovery points were determined offline using a custom Matlab code for the analysis of eye movements. Fusional vergence ranges were also measured with the conventional tests using a Risley prism and a prism bar, and the results were compared.
Results :
When stimulus disparity changed smoothly, narrower BI ranges were obtained with the objective method (mean±SD break of 13.7±4.7PD and 10.6±5.1PD recovery) than with the Risley prism (break of 17.5±3.8PD and 12.8±4.2PD recovery) with a mean±SD difference between methods of -3.8±3.5PD (break) and -2.2±4.7PD (recovery). BO ranges were wider with the objective method (35.4±10.9PD break and 24.4±10.7PD recovery) than with the Risley prism (25.8±7.5PD break and 14.6±5.8PD recovery) with a mean difference of 9.2±10.7PD for the break, and 9.6±10.1PD for the recovery. When disparity changed in steps, fusional ranges measured with the objective method (15.7±5.1PD BI break, 12.8±4.7PD BI recovery, 36.3±11.4PD BO break, 23.9±10.6PD BO recovery) were not significantly different than with the prism bar (14.5±3.8PD BI break, 10.7±3.4PD BI recovery, 34.0±7.8PD BO break, 23.0 ±5.7PD BO recovery). Mean differences between these two methods were 1.2±5.2PD (BI break), 2.1±4.7PD (BI recovery), 2.3±13.0PD (BO break), and 0.9±11.3PD (BO recovery).
Conclusions :
This study showed the possibility to measure fusional vergence ranges objectively. The agreement between the objective method with smooth-changed disparity and the conventional test using the Risley prism was considerably poorer than between the two methods where disparity changed in steps. However, a wide variability across subjects was still found, especially for the BO fusional ranges.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.