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Janis Intoy, Michele A Cox, Michele Rucci; Control and coordination of fixational eye movements in the Snellen acuity test. Journal of Vision 2019;19(10):145a. doi: https://doi.org/10.1167/19.10.145a.
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The ability to resolve spatial details as small as 1-arcmin is a remarkable accomplishment of the human visual system, especially considering that the eyes are always in motion. Humans continually perform fixational eye movements (FEM), consisting of an incessant jitter of the eye (drift) occasionally interrupted by small, rapid gaze shifts (microsaccades). Previous studies have shown that, under monocular viewing conditions, FEM are beneficial for the perception of fine details. FEM both enhance relevant spatial information in the form of temporal modulations and precisely center the stimulus within the fovea, resulting in an overall increment in acuity by 40% in the Snellen test. This is the equivalent of the difference between the 20/30 and the 20/20 lines of an eye chart. Here we investigated oculomotor behavior and coordination during normal binocular execution of a Snellen test. Emmetropic human observers (n=5) identified the orientations of 6 tumbling-E optotypes on the 20/16 line of an eye chart. High-resolution eye movements were recorded by a Dual Purkinje image eye-tracker, while custom gaze-contingent calibration procedures ensured high accuracy in the localization of the two lines of sight. Extending previous monocular results, we report that microsaccades and drift are finely tuned in both eyes. Microsaccades were highly conjugate and jointly moved the two eyes’ lines of sight across optotypes. In the intervals between microsaccades, both eyes drifted significantly slower during the Snellen test relative to when observers were asked to maintain steady fixation on a marker. Drift statistics were highly similar in the two eyes, however, the eyes drifted independently. These findings demonstrate that during normal binocular execution of a high-acuity task, the movements of both eyes are finely tuned. They raise the question of how binocular signals are combined given that the two eyes move independently most of the time.
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