A recent study explored the relationship between eye movement patterns and visual acuity at the level of individual photoreceptor cones in the foveola. Researchers at Rheinische Friedrich-Wilhelms-Universität Bonn used advanced adaptive optics scanning laser ophthalmoscopy to assess how eye drift aids visual acuity, suggesting the eye’s micro-movements help focus light onto densely packed foveal cones, achieving resolution beyond static sampling limits.
More specifically, the high-precision adaptive optics scanning laser ophthalmoscopy (AOSLO) allowed researchers to visualize individual photoreceptor cones in the foveola, which enabled them to measure visual acuity down to a single-cone resolution. They presented a stimulus—Snellen-E optotypes—at different sizes and four orientations of the AOSLO imaging raster. The 20 participants (17 adults and three children) then indicated the stimulus orientation during “natural viewing”, or unrestricted eye movement.
The investigators noted in the article published in eLife, that this was the first time they were able to measure the direct relation between the sampling density of an individual foveolar cone photoreceptor and the patient's visual resolution threshold. The study found a strong correlation between diffraction-limited visual acuity thresholds and foveolar sampling density in both dominant and fellow eyes.
In addition to eye drift, the study observed that microsaccades—small, rapid eye movements—help reposition the foveal cones, ensuring consistent sampling of fine visual details. Contrary to previous beliefs that eye drift is random, the study found that these micro-movements systematically direct light toward areas of the retina with higher cone density and allow the visual system to achieve resolution finer than individual cone spacing.
Participants achieved average visual acuity thresholds that exceeded theoretical static sampling predictions by an average of 18%. Drift length and direction significantly influenced these resolution thresholds with shorter drift lengths correlated with higher acuity. The study also highlighted individual variability in foveal cone density and drift patterns. Higher cone densities correlated with lower visual thresholds, explaining up to 45% of the variance in visual acuity across individuals.
The findings suggest that the combination of eye drift and microsaccades allows the visual system to surpass conventional Nyquist limits of static sampling, effectively enhancing retinal resolution beyond what would be expected from cone spacing alone.
The researchers concluded that recognizing the oculomotor system's ability to precisely adjust drift motion for specific tasks could inform future interpretations of fixational eye movements. They suggested that expanding datasets on photoreceptor-resolved foveolar maps and related visual function metrics could enhance understanding of the relationship between structural and functional changes. This, in turn, could provide insights into disease progression, treatment effectiveness, and the interpretation of retinal imaging in vision restoration studies.
Additionally, they emphasized that detailed psychophysical measurements, combined with precise knowledge of neural sampling, hold significant potential for addressing questions about resolution limits in myopia, the effects of image stabilization at the foveola's center, and implications for binocular vision, which have previously been speculative.