Oculomotor contributions to foveal crowding.

Crowding, the phenomenon of impaired visual discrimination due to nearby objects, has been extensively studied and linked to cortical mechanisms. Traditionally, crowding has been studied extrafoveally; its underlying mechanisms in the central fovea, where acuity is highest, remain de-bated. While low-level oculomotor factors are not thought to play a role in crowding, this study shows that they are key factors in defining foveal crowding. Here we investigate the influence of fixational behavior on foveal crowding and provide a comprehensive assessment of the magnitude and extent of this phenomenon (N=13 human participants, 4 males). Leveraging on a unique blend of tools for high-precision eyetracking and retinal stabilization, we show that removing the retinal motion introduced by oculomotor behavior with retinal stabilization, diminishes the negative effects of crowding. Ultimately, these results indicate that ocular drift contributes to foveal crowding re-sulting in the same pooling region being stimulated both by the target and nearby objects over the course of time, not just in space. The temporal aspect of this phenomenon is peculiar to crowding at this scale and indicates that the mechanisms contributing to foveal and extrafoveal crowding differ.Significance Statement: Foveated stimuli are often crowded. The effects of crowding have been extensively studied in the visual periphery and are thought to have a cortical origin. Nonetheless, foveal crowding mechanisms remain elusive. Here we show that acuity drops by two lines on a Snellen Chart when flankers surround a stimulus presented at the very center of gaze. Further, at this scale, crowding cannot be regarded as a purely cortical phenomenon. Because foveal neurons' receptive fields are the smallest, eye jitter during fixation introduces spatial uncertainty by sweeping target and surrounding distractors over the same cortical pooling region even during short fixation periods, exacerbating crowding effects.