Abstract
Visual sensitivity is transiently attenuated around the time of saccades, a phenomenon known as saccadic suppression. This phenomenon and its origins have been traditionally studied with large saccades and peripheral stimuli, but it is now known that suppression also occurs during small saccades (microsaccades) when stimuli are presented in the fovea. Due to their small amplitude and lower speeds, microsaccades yield little visual masking and retinal smear, the commonly assumed retinal contributors to saccadic suppression, suggesting an important role from possible efference copies associated with microsaccades. Here, we investigate the visual and motor contributions of microsaccades to perceptual suppression within the fovea. We measured contrast sensitivity in a high-acuity task designed after primate social grooming, a task that naturally elicits frequent microsaccades. Observers searched for fleas (targets) amongst dust particles (distractors) embedded in either a naturalistic noise field or uniform background. Targets and distractors were 5 arcmin squares, and targets distinguished themselves by a 10 ms contrast pulse. Subjects either actively searched the targets or passively observed a reconstruction of the visual stimulation recorded during an earlier active trial. A high-resolution system for gaze-contingent display coupled to a digital DPI eye-tracker enabled precise measurement of oculomotor activity in the active condition and its subsequent replay under retinal stabilization in the passive viewing condition. First, we show that a strong suppression occurs at the time of microsaccades regardless of the stimulus background, though a textured background enhances the effect. Second, the dynamics of perceptual suppression are significantly different when the microsaccades are performed or simulated: suppression was delayed and longer-lasting in the passive condition. Therefore, extraretinal modulations first suppress and then enhance sensitivity immediately before and after a saccade. These results show that both the visual consequences and the motor commands associated with microsaccades impact visual sensitivity within the fovea.