Before the eye moves: microsaccade preparation expands spatial integration at the center of gaze

Abstract

Fixation is often treated as a period of stable visual processing. Yet, fixation is often punctuated by frequent microsaccades that occur during tasks involving complex foveal stimuli. These small eye movements are preceded by changes in visual sensitivity, both at the upcoming movement goal and at the currently fixated location. However, previous work has largely focused on isolated stimuli, leaving unclear whether pre-microsaccadic modulations reflect changes in sensitivity alone or also alter the spatial interactions that govern object recognition. Visual crowding provides a direct test of this question because it depends on the integration and segregation of nearby features and constrains recognition even within the foveola. Using high-precision Dual Purkinje Image eye tracking with retinally contingent stimulus delivery, we measured acuity and crowding thresholds at the preferred locus of fixation (PLF), the starting point of the impending gaze shift, while observers either maintained fixation or prepared to execute a microsaccade to a cued location. Unflanked acuity at the PLF remained stable across conditions. In contrast, crowding strength increased during the pre-microsaccadic interval, indicating an expansion of the foveal crowding zone. These results show that microsaccade preparation alters spatial integration at the starting point of the movement, increasing crowding even when sensitivity to isolated stimuli remains unchanged. Thus, microsaccades reshape foveal vision not only by modulating visual discrimination at the movement goal, but also by changing how nearby features are integrated and segregated before the eyes move. Significance StatementVision is often assumed to be most stable when gaze is fixed. Yet the eyes are never truly still, and the brain continually prepares small movements that shape perception before they occur. This study shows that such preparation changes how visual information is organized at the very center of gaze. Upcoming eye movements do not simply alter sensitivity to isolated objects; instead, they change how nearby features are integrated. These findings reveal that fine spatial vision and object recognition depends not only on what falls on the retina or on subsequent cortical processing but also on what the eyes are preparing to do next.