The Impact of Simulated Anisometropia and Bilateral Defocus on Reflex Vergence Responses of Children and Adults.

Purpose: This study assessed the impact of computationally-simulated defocus on reflex vergence movements responsible for maintaining binocularly aligned fixation in adults and children. This question is clinically relevant for uncorrected anisometropia during childhood and the use of monovision to manage presbyopia. Methods: Reflex vergence step and continuous tracking responses were recorded from 28 adults (15-53 years) and 32 children (four to eight years) viewing a small spatially broadband stimulus with disparity changing in steps from 0° to ±4° for 0.32 second or following a random walk for 40 seconds. Defocus of 0, 1, 2 or 4 D was presented bilaterally or unilaterally, simulating anisometropia. Open-loop vergence amplitudes were computed for step trials, and peak correlations and latencies between stimulus and vergence velocities were computed for continuous tracking trials. Results: Vergence responses of adults and children to disparity steps followed typical tuning functions, with amplitudes decreasing with increasing anisometropic defocus (adults: P = 0.002; children: P = 0.042). In the continuous tracking task, anisometropic defocus had a greater detrimental effect on vergence than bilateral defocus (P < 0.001) for both age groups, and unilateral defocus primarily affected movements of the defocused eye. For some participants, this resulted in complete motor dominance of the focused eye, with version movements tracking its stimulus. Conclusions: These results highlight the importance of retinal image similarity for binocular development and motor function. They may help explain the relationship between anisometropia and strabismus during childhood, and the failure of monovision for some patients.