Abstract
High visual acuity is essential for many tasks, from recognizing distant friends to driving a car. While much is known about how the eye’s optics and anatomy contribute to spatial resolution, possible influences from eye movements are rarely considered. Yet humans incessantly move their eyes, and it has long been suggested that oculomotor activity enhances fine pattern vision. Here we examine the role of eye movements in the most common assessment of visual acuity, the Snellen eye chart. By precisely localizing gaze and actively controlling retinal stimulation, we show that fixational behavior improves acuity by more than 0.15 logMAR, at least 2 lines of the Snellen chart. This improvement is achieved by adapting both microsaccades and ocular drifts to precisely position the image on the retina and adjust its motion. These findings show that humans finely tune their fixational eye movements so that they greatly contribute to normal visual acuity.
Highlights
High visual acuity is essential for many tasks, from recognizing distant friends to driving a car
We show that humans actively tune both major components of fixational eye movements, ocular drift and microsaccades, to benefit from the spatial and temporal properties of retinal processing
We coupled high-resolution eye-tracking with real-time control of retinal stimulation to investigate the roles of eye movements in a standard acuity test
Summary
Whether human observers tune them to the task. To this end, we compared the characteristics of the eye movements recorded during inspection of the 20/20 line of a Snellen chart (Fig. 1, Supplementary Movie 1) with those recorded during the initial period of each trial before the appearance of the optotypes, when observers were asked to maintain steady gaze on a fixation marker (a 20 dot). Similar effects were observed when comparing the average microsaccade characteristics recorded in the Snellen test to those measured in the two separate groups of subjects that either maintained fixation for the entire duration of the trial or freely observed natural scenes In both cases, microsaccades were larger than in the Snellen test (90th percentile amplitude in fixation: 33.6 ± 1.6 arcmin; p = 0.0013; free-viewing: 52.7 ± 1.4 arcmin; p < 10−9; ANOVA with post-hoc Tukey–Kramer tests), and they were less likely to shift gaze to the right (percentages of rightwards microsaccades in fixation: 19.6%, p = 0.00003; freeviewing: 15.7%, p < 10−7). Both distances mentioned above remained virtually unchanged when only microsaccades in the Probability a
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