Spatial and Temporal Factors Influencing Fixational Saccades.

Much research has focused on how perceptual, cognitive, and attentional processes modulate microsaccades, the small rapid gaze shifts that humans perform when attempting to maintain steady gaze on a point. Yet the reasons why these fixational saccades occur in the first place have remained unclear. Long-standing theories have argued for either spatial (i.e., gaze centering) or temporal mechanisms (i.e., a periodical release process). However, this debate has never been resolved, primarily because of uncertainty in determining where the observer looks. Whereas modern eye-trackers enable detection of small eye movements, accurate localization of the line of sight remains challenging. Here, rather than indirectly inferring gaze position from oculomotor activity, we used a gaze-contingent procedure to directly estimate the perceived center of the visual field, a method that has been previously shown to effectively reduce uncertainty. Our results from subjects of both sexes show that the generation of fixational saccades depends on the interaction of spatial and temporal factors. Fixational saccades are remarkably accurate in correcting for fixation errors, even when gaze is minimally displaced. However, fixational saccades also occur when gaze is centered, but their latency increases as the fixation error decreases. These results suggest that fixational saccades serve an important corrective function when needed, but they can only be avoided for a limited period of time when fixation is already accurate.