Abstract
Purpose :
The primate oculomotor system is thought to be yoked to generate conjugate eye movements that rotate the eyes symmetrically and keep the foveae aligned. With gaze shifts in depth on the midline, a separate yoked vergence system rotates the eyes through equal but opposite angles. However, eye movements appear to be controlled independently in some types of strabismus. If strabismus is an aberration of normal ocular control, normal binocular eye movements might show hints of independent control. Here we provide evidence that ocular control in normals violates both conjugate and vergence yoking.
Methods :
We measured eye movements and accommodation from each eye simultaneously from six observers with normal or corrected to normal vision, no manifest ocular deviation and normal range of well controlled latent deviation (2 prism diopters esophoria to 6 prism diopters exophoria at near) and no anisometropia (> 0.25 diopter difference). Observers pursued an accommodative target aligned on the midline that moved periodically in depth at an average velocity of 1.3 diopters/sec. The target was viewed either binocularly, or monocularly with one eye occluded by an infrared-passable filter. Eye position and accommodation from each eye was measured simultaneously using a Plusoptix power refractor.
Results :
Surprisingly, covering an eye unmasked pursuit eye movements that violated the assumption of a yoked vergence system, and that differed for each covered eye. One viewing eye appeared “dominant” in that it drove the covered eye conjugately during midline pursuit, while with the other “non-dominant” eye viewing, the covered eye was either virtually immobile or moved with a small conjugate or disconjugate rotation. Furthermore, the observed “oculomotor dominance” did not necessarily correlate with perceptual dominance as assessed by sighting. Interestingly, accommodation was symmetrical when either eye was covered.
Conclusions :
The results suggest that smooth pursuit movements can be controlled asymmetrically, with a single dominant eye driving its fellow without similar changes being seen in accommodation indicating a disconnect between the accommodation-vergence link under monocular conditions. Furthermore, these findings have implications for interactions between cortical and brainstem control of the two eyes, undertsanding loss of control mechanisms and implications for intervention in strabismus.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.