Microscopic eye movements compensate for nonhomogeneous vision within the fovea

Humans rely on the fovea, the small region of the retina where receptors are most densely packed, for seeing fine spatial detail. Outside the fovea, it is well established that a variety of visual functions progressively decline with eccentricity. In contrast, little is known about how vision varies within the central fovea, as incessant microscopic eye movements prevent isolation of adjacent foveal locations. Using a new method for restricting visual stimulation to a selected retinal region, we examined the discrimination of fine patterns at different eccentricities within the foveola. We show that high-acuity judgments are impaired when stimuli are presented just a few arcminutes away from the preferred retinal locus of fixation. Furthermore, we show that this dependence on eccentricity is normally counterbalanced by the occurrence of precisely directed microsaccades, which bring the preferred fixation locus onto the stimulus. Thus, contrary to common assumptions, vision is not uniform within the foveola, but targeted microscopic eye movements compensate for this lack of homogeneity. Our results reveal that microsaccades, like larger saccades, enable examination of the stimulus at a finer level of detail and suggest that a reduced precision in oculomotor control may be responsible for the visual acuity impairments observed in various disorders.