Differences in Preferred Retinal Loci of Fixation in Binocular and Monocular Vision.

PURPOSE: Fixational eye movements (FEMs) are critical for visual perception, yet their precise coordination during binocular viewing remains poorly understood at the microscopic retinal level. This study aimed to quantify differences in fixation stability and the spatial location of the preferred retinal locus (PRL) between monocular and binocular viewing conditions. METHODS: Using a custom binocular scanning laser ophthalmoscope, we recorded high-resolution FEM sequences in 10 healthy subjects under monocular and binocular viewing. Fixation stability was evaluated using the non-parametric isoline contour area (ISOA). A permutation-based statistical framework was applied to account for the strong sample-to-sample correlation in high-speed eye-tracking data. RESULTS: Results show that binocular viewing yields significantly smaller ISOA values compared to monocular viewing (P < 0.001), indicating a binocular advantage in fixation stability. This advantage is driven by a concurrent tightening of both components of fixational eye movements: smaller microsaccade amplitudes (median, 10.04 arcmin vs. 8.11 arcmin; P = 0.0098) and less dispersive drift (P ≤ 0.0039 across time lags of 33-833 ms). Furthermore, we identified systematic, yet individual, intra-subject spatial shifts of the PRL (∆PRL mean, 3.19 ± 2.20 arcmin) upon binocular viewing, which were predominantly driven by a disconjugate micro-vergence. CONCLUSIONS: Binocular viewing not only enhances oculomotor stability through smaller microsaccades and tighter drift but also actively shifts fixation to a different retinal location compared to monocular viewing. The visual system appears to tolerate objective vergence errors of several arcminutes to enable solving the binocular correspondence problem.