Abstract
During normal fixation, the human eyes move incessantly, following seemingly random trajectories. This motion known as ocular drift, has long been assumed to result from imprecise motor control and to be estimated by the visual system from the resulting image motion on the retina. Several lines of evidence now indicate that drift is controlled (Intoy and Rucci, Nat. Comm., 2020). Thus, the question emerges of whether the visual system also has access to extra-retinal information about drift motion. To investigate this question, we designed a task in which extra-retinal information about drift is required for a Vernier judgment. In a 2AFC task, human observers (N=7) reported the spatial relation between Vernier stimuli that were displayed in a gaze-contingent manner as the eye normally drifts. Stimuli were viewed through a thin slit aperture which moved with the eye, enabling exposure of only a vertical strip on the retina, so that the two Vernier bars were vertically aligned on the retina. Thus, the retinal image contains no Vernier offset, but a percept of displacement could arise if the subject has access to extra-retinal knowledge of the direction of ocular drift between the flashes. Special care was taken to eliminate all spatial cues, by showing stimuli in complete darkness and using a custom-developed LED display with no visual persistence. All subjects were able to accomplish the task with Vernier gap of just 2’ and exhibited higher performance as the gap increased. Data are well predicted by a Bayesian model that incorporates prior knowledge of drift motion as a Brownian motion process. These results indicate that humans have access to extraretinal drift information and use it in establishing spatial representations.