Abstract
Purpose
Macular degeneration (MD) is associated with a loss of stereovision, dizziness, and instability. Though experiments looking at vestibular function in this population are limited, one study showed vestibuloocular reflex (VOR) gains that were higher than predicted for near viewing (0.57 m, Gonzalez et al. 2018). Generally, 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). We hypothesized that loss of central vision, binocular scotoma asymmetries, and loss of stereovision may lead to an inability to appropriately adjust the VOR gain to near viewing.
Methods
We examined VOR responses and vergence angles at three viewing distances (point of fusion, 0.5, & 2m) in 7 individuals with maculopathy (61-82, 5F) and 4 age-matched controls (60-81, 4F). Participants fixated a static LED target while sinusoidally rotating their heads in the horizontal plane at 0.5Hz, to the sound of a metronome. An infrared binocular eye tracker (EyeSeeCam Sci) was used to record eye and head movements. All participants were screened for history of vestibular dysfunction and had their stereoacuity tested using the Randot test.
Results
Participants with and without maculopathy exhibited appropriate VOR gain adjustments for near and far viewing (Figure 1A, compare individual points to predicted values: black lines). Both groups had vergence angles that were predictive of VOR gain (Figure 1B) but greater than ideal for near viewing (≤0.5 m, Figure 1C).
Conclusions
Contrary to our hypothesis, we find that adjustments in VOR gain and vergence angle with viewing distance in individuals with MD are similar to those for age-matched controls. However, we do not observe higher-than-predicted VOR gains at near viewing in CFL, as reported previously. This discrepancy may be due to our aggregate analysis of leftward and rightward head movements, though we do not observe VOR gain asymmetries, even in individuals with greatest viewing eccentricities. Our findings suggest that foveation, precise binocular alignment, and stereovision may not be essential for VOR gain adjustments to viewing distance.