Our capacity for vision begins with the light-sensitive photoreceptor cells located in our eyes. A distinct area of the retina, known as the fovea, plays a crucial role in providing clear vision. Within this region, the color-sensitive cone photoreceptors enable us to notice even the tiniest details. The concentration of these cells can differ among individuals. Moreover, when we focus on an object, our eyes perform slight, ongoing movements that also vary from person to person. A team of researchers from the University Hospital Bonn (UKB) and the University of Bonn has explored how these minute eye movements relate to our clear vision and the arrangement of cones. By utilizing high-resolution imaging and micro-psychophysics techniques, they showed that eye movements are finely calibrated to optimize the sampling by the cones. The outcomes of this research have recently been published in the journal eLife.
Humans achieve clear vision by directing their gaze onto an object, thanks to a small section in the center of the retina called the fovea (from Latin, meaning “pit”). This area contains a densely packed arrangement of light-sensitive cone photoreceptor cells, with densities exceeding 200,000 cones in each square millimeter — all within an area roughly 200 times smaller than a quarter-dollar coin. These tiny foveal cones sample the visual field and transmit signals to the brain, resembling the pixels of a camera sensor that are spread across its surface.
However, a key difference exists: unlike camera sensor pixels, the cones in the fovea are not uniformly arranged. Each person’s eye has a distinct density pattern within the fovea. Additionally, “in contrast to a camera, our eyes are constantly and unconsciously moving,” explains Dr. Wolf Harmening, the head of the AOVision Laboratory at the Department of Ophthalmology at UKB and a contributor to the Transdisciplinary Research Area (TRA) “Life & Health” at the University of Bonn. This continual motion occurs even when we are steadily gazing at a stationary object. These fixational eye movements convey intricate spatial details by introducing ever-changing signals from photoreceptors, which the brain must interpret. It is well recognized that one aspect of these fixational movements, known as drift, can differ from person to person, and more significant eye movements can hinder vision. However, the relationship between drift and the foveal photoreceptors, as well as our capacity to discern fine details, has not been thoroughly studied until now.
Utilizing Advanced Imaging and Micro-Psychophysics
This is exactly what Harmening’s research team has examined using an adaptive optics scanning light ophthalmoscope (AOSLO), the only device of its kind in Germany. This instrument’s exceptional precision enabled the researchers to analyze the direct correlation between the density of cones in the fovea and the fine details we can perceive. Simultaneously, they captured the minute movements of the participants’ eyes. To achieve this, they assessed the visual acuity of 16 healthy individuals engaged in a visually intensive task. The team monitored the movement of visual stimuli on the retina to later identify which photoreceptor cells contributed to each participant’s vision. The researchers, including first author Jenny Witten from the Department of Ophthalmology at UKB and a PhD student at the University of Bonn, employed AOSLO video recordings to investigate how the participants’ eyes moved during a task involving letter discrimination.
Eye Movements Are Precisely Tuned to Cone Density
The study concluded that humans can perceive finer details than the cone density in the fovea might predict. “This suggests that the spatial arrangement of foveal cones only partly accounts for resolving acuity,” reports Harmening. Furthermore, the team discovered that slight eye movements significantly impact sharp vision: during fixation, drift movements of the eyes are meticulously coordinated to shift the retina in line with the foveal cone structure. “These drift movements repeatedly directed visual stimuli toward the area with the highest cone density,” Witten explains. Overall, the findings revealed that within just a few hundred milliseconds, drifting behavior adjusted to retinal zones with greater cone density, enhancing sharp vision. The length and orientation of these drifting movements were vital factors.
According to Harmening and his team, these findings offer new perspectives on the fundamental connections between eye physiology and vision: “Understanding how the eye moves optimally to achieve clear vision can enhance our knowledge of ophthalmological and neuropsychological disorders, as well as improve technological solutions aimed at replicating or restoring human vision, such as retinal implants.”
Funding: This research was funded by the Emmy Noether Program of the German Research Foundation (DFG); the Carl Zeiss Foundation (HC-AOSLO); Novartis Pharma GmbH (EYENovative research award); and the Open Access Publication Fund of the University of Bonn.
