Abstract
Children with anisometropia (unequal refractive error in the two eyes) are at risk for atypical binocular development. Here we assessed the effect of unilateral and bilateral spherical defocus on reflex vergence continuous tracking responses. Typical adults (N=18, 16-53 years) viewed a 2deg cartoon displayed dichoptically on a gray background. Spherical defocus of 1, 2 or 4D was applied to one or both eyes’ stimuli using convolution with the Point Spread Function for the subject’s pupil size. Eye movements were recorded with an Eyelink 1000 (SR Research) while retinal disparity followed a random walk in velocity for 40s. Cross-correlations between stimulus and vergence velocities were performed and peak correlations >0.4 were considered significant (95% CI for correlations between random stimuli). With no defocus, 94% of trials showed significant correlations with a median (IQR) peak value of 0.61 (0.14). With 4D of bilateral defocus, 75% of peak correlations remained >0.4 (median=0.57, IQR=0.23). This percentage decreased to 42% with increasing unilateral defocus (peak correlations of 0.64 (0.11), 0.49 (0.20), 0.33 (0.41) with 1, 2 and 4D, respectively). The median (IQR) latency with no defocus of 0.183 s (0.04 s) increased to 0.198 s (0.04 s) and 0.223 s (0.04 s) with 4D of bilateral and unilateral defocus, respectively. As the amount of unilateral defocus increased, some subjects exhibited periods of tracking the focused stimulus with version movements. A consistent increase in version peak correlation was seen from 0 (0.05) with no defocus to 0.22 (0.33) with 4 D of unilateral defocus. This behavior did not occur with bilateral defocus (0.01 (0.13) with 4D). These results demonstrated a more detrimental effect of unilateral than bilateral defocus on vergence performance. Moreover, the periods of version movements reveal how anisometropia may compromise reflexive eye alignment.