Abstract
Before the eyes begin to move, vision is briefly enhanced at the saccade goal, followed by a plummet in sensitivity as the eyes relocate the center of gaze. Recent work showed that vision is also enhanced foveally in a similar fashion before the onset of microsaccades. Here we examine these selective modulations of foveal vision at a finer temporal grain before, during and after the onset of microsaccades. Fixational eye movements were monitored with a high-resolution DPI eye-tracker while subjects performed a high-acuity discrimination task. Human observers (N=4) shifted their gaze with microsaccades to one of two foveal locations (20’ away) based on a preceding a saccade cue. High-acuity stimuli (7’x3 titled bars’) were then briefly presented at varying times relative to microsaccade onset. A response cue appeared after microsaccade execution and subjects reported the orientation of the stimuli previously presented at the location indicated by this cue. Our findings show that stimuli presented at the microsaccade goal were perceptually enhanced. The extent of this enhancement was such that discrimination performance at this location equated performance when stimuli were presented at the preferred locus of fixation (82%±0.1% vs 85%±0.1%, p=0.75). On the other hand, sensitivity at isoeccentric locations opposite to the microsaccade goal, was impaired compared to baseline (61%±0.1% vs 78%±0.1%, p<0.01). This modulation started ~150ms prior to the microsaccade onset. High acuity vision was severely impaired if stimuli were presented when the eyes were in flight. However, within 50 ms from microsaccade landing perception rapidly recovered back to baseline. These findings show that foveal vision drastically changes around the time of microsaccades, and that pre-microsaccadic modulation of foveal vision equates fine spatial vision at the microsaccade goal to that at the preferred locus of fixation, where acuity is highest.