How Gaze Direction and Dynamics Affect Visual Resolution.

Humans exhibit machine-like eye movements (consistent and repeatable) in space and time while performing demanding acuity tasks. To investigate these, we used an adaptive optics imaging and display system in 6 human subjects (4 females, 2 males) to present ultra-sharp Vernier acuity stimuli briefly every 2 s while simultaneously measuring eye movements, including precisely where on the retina each stimulus fell. We found that drifts and microsaccades combined to confine the landing location of the anticipated stimulus to a tiny retinal region centered on the preferred retinal locus (PRL). The variance of landing location was smallest at the time of stimulus presentation and a few hundred milliseconds after. We correlated where the stimulus fell in space and time with correct or incorrect responses. The PRL and a small area around it, including the anatomical fovea, conferred the best acuity. Acuity declined consistently in the rare events in which the stimulus fell more than 5 minarc from the PRL. We also found that acuity was best when the last microsaccade occurred sufficiently prior to stimulus presentation. Our findings reveal a highly evolved oculomotor system where gaze direction during fixation is rarely far enough from the PRL to compromise visual resolution when a person makes natural fixational eye movements.