Effect of Viewing Distance on the Vestibuloocular Reflex in Central Field Loss

Age-related macular degeneration (AMD) can often lead to the loss of the fovea and the surrounding central visual field. This type of visual loss is extremely common (affecting nearly 7% of individuals over 40 in the United states alone (Klein et al. 2011), and are associated with an increase in vestibular complaints. In fact, two thirds of patients with central field loss (CFL) complain of dizziness and instability, and have an increased rate of falls, injury, oscillopsia, and a fear of falling. For certain tasks, individuals develop a new, eccentric fixational area – the preferred retinal locus (PRL). Data on vestibular function in this population are sparse. One study showed a potential re-referencing of the vestibuloocular reflex (VOR) to the eccentric PRL during near viewing and in darkness, with participants with AMD showing an oculomotor asymmetry in the direction of the PRL (González et al. 2018). The study also suggested that VOR gains were also significantly higher in AMD than controls, especially in the near viewing condition. In general, changes in VOR gain with viewing distance are proportional to the vergence angle, with greater vergence angle corresponding to higher VOR gains (exceeding 1 for near viewing). However, there is evidence suggesting that the relationship is not causal (Synder et al. 1992). Thus, VOR gains appropriate to the viewing distance may be preprogrammed over the life span and independent of the actual oculomotor mechanics at the time of testing. Therefore, although visual field defects are known to affect stereopsis and vergence eye movements, we hypothesized that individuals with CFL would exhibit appropriate changes in VOR gain given viewing distance. To test this hypothesis, we examined VOR responses in 5 individuals with CFL (57-76, 3M) and compared them to 8 age-matched controls (50-77, 5M). Participants viewed a static 1° target on a large screen while volitionally moving their heads in a sinusoidal motion in the yaw plane. Movement frequency was controlled using a metronome and participants were instructed on the amplitude of the movement prior to the experiment and were allowed several practice cycles. Experiment was repeated at viewing distances of 50 (near) and 150 (far) cm. Eye and head movements were recorded using a head-mounted, infrared binocular eye tracking goggles (PupilLabs) with an inertial measurement unit (LPMS Research) rigidly attached to the frame of the eye tracking goggles. All participants were screened for history of vestibular dysfunction and had their stereoacuity tested using the Randot test. Microperimetry was performed on all CFL participants to determine size of field defect and fixational locus location. We found that both individuals with central field loss and age-matched controls exhibited appropriate VOR gains for near and far viewing (Figure 1, compare individual points to predicted values indicated by dashed lines) with no significant difference in VOR gain between participant types (p = 0.15). We did find, however, a participant type-by-viewing-distance interaction, suggesting that the effect was significantly greater for control participants than those with CFL (Figure 2, p = 0.02). Our data do suggest changes in vergence angle across groups. Our findings are consistent with our hypothesis that changes in VOR gain with viewing distance can occur in individuals with central field loss and altered oculomotor dynamics. However, we do not observe difference in VOR gain at near viewing in CFL, as reported previously. This discrepancy may be due to our aggregate analysis of leftward and rightward head movements. However, we did not observe VOR gain asymmetries, even in individuals with greatest viewing eccentricities.