Abstract
When saccades are generated, the retinal motion that would dominate the visual input during the eye movement is not perceived. Instead, post-saccadic percepts shift backwards in time—pre-dating the new fixation to before saccade onset (Hunt & Cavanagh, 2009). We investigated whether saccades must be voluntarily initiated and consciously controlled for this temporal expansion to take effect, and probed the visual system’s ability to estimate the timing of stimuli during microsaccades—a very fast type of spontaneous, unconscious miniature eye movement taking place during intended fixation. We displayed a vertically oriented, high-contrast Gabor (4 cpd) and shifted its phase at high temporal frequencies (>60 Hz). This internal stimulus motion rendered the Gabor invisible against a gray background during stable fixation. However, microsaccades that matched the phase shift’s direction and speed briefly stabilized the Gabor on the retina and could therefore lead to its detection. Observers reported the time point of stimulus detection by adjusting a clock hand at the end of the trial, allowing us to determine when a stimulus was perceived. Observers reliably detected the Gabor during microsaccades: Visual sensitivity depended on the match of the microsaccade’s to the phase shift’s velocity and direction, increasing with larger similarities between these metrics. Replaying the retinal consequences of an observer’s previous microsaccades bore out remarkable similarities to these findings, suggesting that visual sensitivity during microsaccades is not limited per se. Interestingly, temporal adjustments revealed that 7 out of our 10 observers reported stimuli earlier when seen during self-generated compared to replayed microsaccades. Our data thus indicate that correlates of active microsaccade generation are used for the temporal processing of stimuli even in the absence of conscious, volitional control of the eye movement: The generation of microsaccades informs the timing of visual events, pre-dating it relative to the arrival of sensory input.