Eye drift during fixation predicts visual acuity

Abstract

Visual acuity is commonly assumed to be determined by the eye optics and spatial sampling in the retina. Unlike a camera, however, the eyes are never stationary during the acquisition of visual information, a jittery motion known as ocular drift, incessantly displaces stimuli over many photoreceptors. Previous studies have shown that acuity is impaired in the absence of retinal image motion caused by eye drift. However, the relation between individual drift characteristics and acuity remains unknown. Here we show that (a) healthy emmetropes exhibit a large variability in their amount of drift; and (b) that these differences profoundly affect the structure of spatiotemporal signals to the retina. We further show that (c) the spectral distribution of the resulting luminance modulations strongly correlates with individual visual acuity; and (d) that natural inter-trial fluctuations in the amount of drift modulate acuity. As a consequence, in healthy emmetropes acuity can be predicted from the motor behavior elicited by a simple fixation task, without directly measuring it. These results shed new light on how oculomotor behavior contributes to fine spatial vision. SignificanceHealthy humans can visually resolve extremely fine patterns, in some cases with the relevant features spanning less than a single photoreceptor on the retina. This accomplishment is particularly remarkable considering that the eyes are never stationary. Ocular drift--a motion that eludes human awareness--shifts the stimulus across many photoreceptors during the acquisition of visual information. Here we show that visual acuity depends on ocular drift. Natural variations in the amount of drift are associated with acuity both within and across subjects, so that individual acuity limits can be directly inferred from the amount of motion during fixation on a marker. Results closely follow the strength of the luminance modulations caused by ocular drift, providing support to long-standing dynamic theories of visual acuity.