Abstract
We previously showed that before the onset of a microsaccade, fine spatial vision is enhanced at the microsaccade goal. In this study, we investigated the spatial resolution of this phenomenon and examined to which extent this enhancement spreads to foveal locations surrounding the microsaccade target. In addition, we examined how microsaccade preparation impacts sensitivity at the preferred locus of fixation where fine spatial vision is highest. Observers (n = 7) fixated on a marker surrounded by eight location placeholders presented foveally and arranged in a circle (20’ radius). Observers were then instructed to shift their gaze at the location indicated by a central saccade cue. Subjects naturally used microsaccades to relocate their gaze. Nine probes (7’x2’ tilted bars) were briefly presented, one at each location and one at the center of gaze, before the onset of the microsaccade. After microsaccade landing, a response cue appeared. Observers reported the orientation of the probe previously presented at the location indicated by the response cue. Our findings show that pre-microsaccadic enhancements of sensitivity are limited to the microsaccade goal: decrease in performance was observed at the locations surrounding (7’ edge-to-edge distance) the microsaccade goal (1.9 vs. 0.4 and 0.3 d’, p < 0.002). Furthermore, before microsaccade onset, observers’ sensitivity at the center of gaze decreased by more than half compared to baseline. We also observed a vertical vs. horizontal meridian asymmetry in foveal sensitivity: observers’ performance at baseline was higher for stimuli presented at locations along the horizontal than the vertical meridian (p < 0.002). This asymmetry, however, was reduced right before the onset of microsaccades. These findings show that pre-microsaccadic enhancements of fine spatial vision are highly localized in the foveola, and that microsaccade onset is preceded by a drop of sensitivity at the preferred locus of fixation.