Abstract
Purpose :
Chandna et al. (2021) recently showed that during monocular pursuit of a midline target, the covered eye usually did not execute a normal vergence response. Rather, it tended to make a conjugate eye movement while accommodation showed a symmetric response. Although accommodation, driven by blur and proximity, and through vergence by disparity cues, is normally thought to be yoked, there is some evidence for decoupling; e.g. aniso-accommodation reported in anisometropic amblyopia in children (Toor et al., 2018) and experimentally lens-induced in adults (Marran & Schor, 1998). We investigated whether manipulating cues driving accommodation and vergence in a binocular or monocular midline ocular tracking task can decouple accommodation responses in the two eyes and provide clues to the underlying mechanism.
Methods :
The data from an earlier study of midline vergence that did test blur and proximity, along with disparity (Horwood and Riddell, 2008) were re-examined. 22 typical participants (age range 19-25 years) with normal visual acuity and binocular status were included. Subjects were naïve to the task and inexperienced with vision experiments, and instructed to observe a midline target (clown image or Gabor patch) under eight different paradigms. Each of these paradigms involved the manipulation of the target to add or remove cues for blur, proximity or disparity.
Results :
In paradigms where there was no disparity cue (i.e., monocular viewing) the covered eye still executed predominantly conjugate rather than vergence movements (Horwood et al., 2001; Chandna et al., 2021). There was also evidence of decoupled accommodation for monocular viewing. The covered eye had a significantly lower accommodation amplitude than the viewing eye (p < 0.01; Friedman test, Tukey-Kramer correction). The asymmetric accommodation was not present with binocular viewing even when blur and proximity cues were removed (p > 0.01; Friedman test, Tukey-Kramer correction).
Conclusions :
The results suggest that the accommodation response is not as strongly yoked as previously believed. Absence of the disparity cue results in asymmetric accommodation and decoupling of the accommodative response as well as the vergence response. This may explain the asymmetric accommodation observed in previous studies as well as that observed in anisometropia, in addition to variability in AC/A ratios in certain types of strabismus.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.