Abstract
The visual functions of microsaccades, the microscopic saccades that humans perform while attempting to maintain fixation, have long been debated. The traditional proposal that microsaccades prevent perceptual fading has been criticized on multiple grounds. We have recently shown that, during execution of a high-acuity task, microsaccades move the gaze to nearby regions of interest according to the ongoing demands of the task (Ko et al., Nature Neurosci. 2010). That is, microsaccades are used to examine a narrow region of space in the same way larger saccades normally enable exploration of a visual scene. Given that microsaccades keep the stimulus within the fovea, what is the function of these small gaze relocations? By using new gaze-contingent display procedures, we were able to selectively stimulate retinal regions at specific eccentricities within the fovea. We show that, contrary to common assumptions, vision is not uniform within the fovea: a stimulus displacement from the center of gaze of only 10 arcmin already causes a significant reduction in performance in a high-acuity task. We also show that precisely-directed microsaccades compensate for this lack of homogeneity giving the false impression of uniform foveal vision in experiments that lack control of retinal stimulation. Finally, we show that the perceptual improvement given by microsaccades in high-acuity tasks results from accurately positioning the preferred retinal locus in space rather than from the temporal transients microsaccades generate. These results demonstrate that vision and motor behavior operate in a closed loop also during visual fixation.
Meeting abstract presented at VSS 2013