"Use and Placement of Fovea in Smooth Pursuit"

Abstract

Foveation during smooth pursuit has been a topic of debate for many decades. Often thought of as an oculomotor behavior designed to maintain visibility and acuity on a moving target, smooth pursuit has also been modeled as a velocity-driven system that is agnostic of target position. In this talk, I will discuss data from two recent publications that directly address this debate. Using a novel technique of measuring monocular smooth pursuit while visualizing the retina in a scanning laser ophthalmoscope (SLO), we examined retinal target placement strategies during smooth pursuit first in a group of individuals who have lost foveal vision due to macular degeneration, and subsequently in a cohort of healthy controls. In the first study, we found that individuals who did not have a functional fovea were able to pursue a moving target, although they did not use the locus they used for fixating a static target. In the second study, we found the same behavior in individuals with healthy foveas —they did not track the moving target with their functional fovea on a majority of trials. More often, participants placed their fixational locus on a more eccentric portion of the target. We also compared fovea placement during pursuit to that during saccades. In contrast to pursuit, healthy participants tended to foveate the center of mass of the stimulus during saccades, with a hypometric bias. Thus, our data provide the first direct evidence that foveation is not common during smooth pursuit, even when the moving stimulus is a single spot. Furthermore, it suggests that foveal placement strategies are fundamentally different during smooth pursuit compared to saccades.