Gaze Changes from Binocular to Monocular Viewing during Smooth Pursuit in Macular Degeneration

Purpose: The asymmetrical nature of bilateral central field loss in macular degeneration (MD) makes viewing strategies difficult to predict. Previous work examined gaze shifts from monocular to binocular fixation and found that the position change was negligible for the better eye, but significant for the worse eye (Kabanarou et al. 2006). Here, we set out to determine if analogous gaze shifts occurred when switching from monocular to binocular smooth pursuit. Methods: Participants (6 MD, 2 age-matched controls) were asked to pursue a 1° circular target in a step ramp paradigm (Rashbass 1961), moving in 6 possible directions at 5, 10 and 15°/s. Viewing was either binocular or monocular, and eye movements of both eyes were recorded (EyeLink 1000) with the non-viewing eye occluded with an infrared filter. To determine gaze changes during fixation, we averaged eye position during pre-pursuit segments across all trials and computed differences in eye position between binocular and monocular viewing for each eye. To determine whether gaze position is different between binocular and monocular viewing during smooth pursuit, we looked at the last 100 ms of each trial, averaging per direction and velocity condition. Results: During the pre-pursuit, static phase, individuals with MD had a more variable fixation strategy than in Kabanarou et al. (2006). Across participants, 43% of trials showed a significant gaze shift between monocular and binocular viewing in the horizontal or vertical direction. During pursuit, gaze shifts between monocular and binocular conditions depended on target direction for all patients, which may be related to the dependence of smooth pursuit gain on target direction and scotoma location (Shanidze et al., in press). Despite this variability, eyes that stayed within the BCEA when the target was static were more likely to have a smaller binocular-to -monocular position shift: for trials with a large shift during fixation, 83% also had a large shift during pursuit; for trials with a negligible shift during fixation, 50% had a negligible shift during pursuit (Wilcoxon Rank-Sum test, p<0.0001). Conclusions: For individuals with MD, gaze shifts from monocular to binocular viewing during fixation predict gaze shifts during dynamic tasks for 2/3 of trials. For the remainder of the trials, eye position may depend on other factors, such as gaze direction and target motion relative to scotoma.