Oculomotor influences on the dynamics of visual sensitivity

Abstract

Humans continually move their eyes, alternating saccades with a smooth fixational motion known as ocular drift. The saccade/drift cycle modulates the visual input to the retina in a highly specific manner, delivering spatiotemporal signals with power that shifts from low to high spatial frequencies during the course of post-saccadic fixation. Recent research has shown that these signals contribute to a coarse-to-fine dynamics of contrast sensitivity when stimuli are presented in the central visual field. Here we show that the saccade/fixation cycle carries similar perceptual consequences across eccentricities. We measured contrast sensitivity to low and high spatial frequencies (2 or 10 cycles/deg) at three visual eccentricities (0, 4 and 8) and various delays (50, 150, or 500 ms) following an instructed saccade (6.6 degrees). Subjects were asked to report the presence/absence of a circular grating embedded within a naturalistic noise field, as their eye movements were recorded by a digital DPI eyetracker. To elicit a normal saccade transient while preventing visibility of the grating before the saccade, the grating appeared at saccade onset and remained on the display for a fixed interval following its offset. Sensitivity to low spatial frequency was high immediately following the saccade and uniform across eccentricities. Continued exposure during fixation provided minimal improvement to sensitivity. In contrast, sensitivity to high spatial frequency declined with increasing eccentricity, as expected. However, sensitivity improved with fixation duration at similar rate at all eccentricities. To examine the origins of these effects, we exposed models of the retinal ganglion cells (magno- and parvo-cellular, ON and OFF) to reconstructions of the visual input signals experienced by subjects in the experiments. A standard decision-making model that cumulated responses over the saccade/fixation cycle accurately replicated visual dynamics. Dissection of the model shows that the oculomotor-shaped luminance dynamics is primarily responsible for these effects.