The Impact of Sub-Foveal Scotomas on Visual Perception and Fine Oculomotor Behavior

It is known that in the presence of extensive foveal vision loss, the oculomotor system re-references saccades to a new preferred locus of fixation outside the affected area. However, in most circumstances, visual degeneration in the fovea is progressive and starts with more localized losses before symptoms arise. It remains unknown what happens in these early stages, and if fine oculomotor behavior is influenced by these losses. Here, we investigated whether sub-foveal vision loss has an impact on perception, and, if and how this loss alters fine oculomotor behavior. An artificial sub-foveal scotoma just a few arcminutes in size (a gray gaussian blob with a standard deviation of 5 arcminutes) was rendered using a gaze-contingent display (360 Hz refresh rate) coupled with a high-precision eye-tracker enabling arcminute accuracy in the localization of the line of sight. Visual stimuli were viewed normally and in the presence of the artificial scotoma retinally stabilized at the preferred locus of fixation. We examined subjects’ (N=5, normally sighted) performance when discriminating complex foveal stimuli, and assessed systematic changes in their oculomotor behavior during sustained fixation on a 7x7 arcminutes marker. Our results show that the artificial sub-foveal scotoma induced a 5% drop in visual discrimination (p = 0.035). Additionally, during sustained fixation, we report an idiosyncratic but consistent offset of the preferred locus of fixation (p = 0.009, median offset 8 ± 0.9 arcminutes), large enough to place the fixation marker outside the edge of the scotoma. These findings indicate that even a tiny sub-foveal scotoma impacts performance and leads to fine adjustments of the preferred locus of fixation, suggesting that fixational eye movements can potentially play a role in the early stages of foveal vision loss.