Maximilian Freiberg^1,2 (), Aleksandr Gutnikov¹, Christian Meltendorf², Stephan Reiß², Ralph Krüger², Wolf M. Harmening¹; ¹University Eye Clinic Bonn, ²Berlin University of Applied Sciences and Technology

To determine the preferred retinal locus of fixation under monocular and binocular fixation conditions and to quantify potential differences regarding location and stability across conditions. The retinal locus of fixation of both eyes of 6 participants with drug induced pupil dilation and of 10 participants without pupil dilation were determined by imaging the central 3x3° of the retinae with high-resolution split-field binocular scanning laser ophthalmoscopy. Five videos, 10 seconds each, were recorded for each participant during monocular and binocular fixation. Participants were instructed to fixate a stimulus (6 arcmin2 black square) centered in the imaging raster as accurately and relaxed as possible. Fixational eye movements were extracted by strip-wise image registration of the video material. The retinal fixation loci (PRL) were determined as the median x- and y-coordinate of all registered gaze positions. Gaze changes (PRL offsets) between viewing conditions were quantified as their euclidean distance. Fixation stability was determined by computing the iso-contour area (ISOA) of all retinal landing points via probability density estimation. Across participants, PRL offsets between monocular and binocular fixation were found to be between 0.98 - 10.44 minutes of arc with significant differences (p < .05, paired t-test) in 6 of 10 (without pupil dilation) and in 5 of 6 participants (with pupil dilation). Fixation stability was significantly higher in binocular conditions across the cohort without pupil dilation (p < .05, paired t-test). Average binocular ISOA was 62.01 (±28.83) arcmin2 in right eyes and 72.72 (±26.27) arcmin2 in left eyes, and 107.43 (±58.49) arcmin2 in right eyes and 114.52 (±37.09) arcmin2 in left eyes during monocular fixation. Small but significant inter-individual differences of the PRL in monocular versus binocular vision were observed which points to normal but not exact binocular interaction during fixation. Fixation stability was about twice as high during binocular vision.

Acknowledgements: Funding: WMH: German Research Foundation (DFG Ha5323/5-1), Dr. Eberhard und Hilde Rüdiger Stiftung (binoSLO)