Abstract
Purpose :
To analyse and characterise the saccades that occur during the fusional vergence test as a function of the disparity vergence demand.
Methods :
Participants’ BO (base out) and BI (base in) near fusional vergence ranges were measured in an haploscopic set-up and recording eye movements with an Eyelink 1000 Plus (SR Research) at 500Hz. The visual stimulus was a column of letters of 0.20 LogMAR VA, and its disparity changed smoothly at 1PD/s up to 45PD for both divergence (BI) and convergence (BO). Break and recovery points were determined objectively offline using a custom Matlab code for the analysis of vergence movements. Saccades were detected with the velocity-threshold-based method of Engbert-Kliegl (2003).
Results :
The study involved 34 adults with normal binocular vision (mean age±SD 23.2±2.4 years). A total of 13103 and 14381 saccades were detected during the measurement of BI and BO fusional vergence ranges, respectively. Saccades followed the main sequence (R=0.97, p<0.001), and 47.65% had amplitudes <1deg. The distributions of saccadic amplitudes during convergence and divergence differed significantly (U=4.28, p<0.001). The amplitude of saccades that occurred while fusion was maintained [median (IQR) 0.73 (0.92) deg] was significantly smaller than that of saccades during diplopia [2.10 (3.90) deg] (U=-75.63, p<0.001). Vertical saccades (directions within ±22.5deg from vertical) were more prevalent (39%) than horizontal (directions within ±22.5deg from horizontal) ones (31%) during the measure of BO fusional ranges, while the opposite was observed during the measure of BI ranges (52% horizontal and 22% vertical). Participants experienced diplopia for a longer time during the measure of BI fusional ranges compared to BO, as the stimulus disparity always reached 45PD regardless of the break point. This might explain the effect of vergence sign on saccade direction, as participants tended to alternate fixation between the two diplopic images, whereas with fusion, they tended to make more vertical saccades reading the column of letters.
Conclusions :
The study provides insight into the oculomotor behaviour during the fusional vergence test and concludes that saccades characteristics are influenced by binocular conditions. These findings contribute to a better understanding of how the visual system adjusts saccades in response to different disparity vergence demand during fusional vergence range evaluation.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.