Systematic variation of fixational eye movements with degree of myopia.

Studies of myopia and emmetropization traditionally focus on the spatial quality of visual input, as if the eyes were immobile. However, the input to the retina is never stationary. Even when fixating steadily on an object, the eyes continually jitter in an ostensibly erratic manner. This motion, known as ocular drift, shifts the image on the retina across many photoreceptors, reformatting visual input signals according to their spatial frequencies. The resulting luminance modulations increase in amplitude up to a spatial frequency limit that depends on the amount of drift. Recent studies with emmetropes have shown that (a) humans actively control drift in ways consistent with taking advantage of this input reformatting; and (b) individual acuity limits are strongly correlated with their idiosyncratic drift characteristics. Here we examined ocular drifts in myopic observers (-0.25D to -6.5D; N=19) during two high-acuity tasks: identification of the tumbling E optotypes in a 20/20 line of Snellen eye chart, and discrimination of the gaze direction of distant faces (1⁰; looking straight vs. looking away). Stimuli were observed monocularly while refractive errors were corrected by a Badal optometer. Refraction (sphere and cylinder), corneal curvature, and eye length were measured using standard methods, and eye movements continually recorded at high resolution via Dual Purkinje Imaging. We found that ocular drift changes systematically with the degree of myopia: its speed increases (p = 0.004) whereas its curvature decreases (p = 0.01) with increasing eye length. These changes redistribute the strength of luminance modulations toward lower spatial frequencies. Thus, eye movements qualitatively act like the low-pass filters (e.g., diffusers) used to induce myopia in the laboratory. These findings highlight the importance of considering oculomotor behavior in the context of eye development and myopia.