Study: Subtle eye movements optimize vision

Study first author Jenny Witten is shown in front of the micro-psychophysical setup. (Image credit: MIB-Center at the UKB / Volker Lannert)

Our ability to perceive visual detail begins with the photoreceptor cells in our eyes. A specialized region of the retina called the fovea is responsible for high-acuity vision, and within it, color-sensitive cone photoreceptors enable us to distinguish fine details.

The density of these cones varies between individuals, influencing visual resolution. Additionally, our eyes make minute, continuous movements—differing in pattern and size across individuals—when focusing on a single object. Researchers from University Hospital Bonn (UKB) and the University of Bonn have examined how sharp vision is influenced by these small eye movements and the foveal cone mosaic.

By utilizing high-resolution imaging and micro-psychophysics, they demonstrated that eye movements are calibrated to optimize sampling by cones. Their findings were recently published in eLife^.1

Humans achieve clear vision by fixating on an object through the fovea, a small pit in the retina densely packed with cone photoreceptors. This mosaic of cones reaches densities exceeding 200,000 per square millimeter, within an area roughly 200 times smaller than a quarter. These foveal cones sample the visible field and transmit signals to the brain, functioning similarly to pixels in a camera sensor with millions of photosensitive elements. Unlike camera pixels, however, cone distribution in the fovea is non-uniform, and each individual’s fovea has a unique density arrangement.

“Unlike a camera, our eyes are constantly and unconsciously in motion," explained Wolf Harmening, PhD, head of the AOVision Laboratory in the Department of Ophthalmology at UKB and a member of the Transdisciplinary Research Area “Life & Health” at the University of Bonn. ²

This movement occurs even when fixating on a stationary object. These fixational eye movements introduce slight variations in photoreceptor stimulation, which the brain interprets to refine spatial details. Drift, a component of these fixational movements, varies among individuals, and excessive movement can impair vision. Until now, the relationship between drift, foveal cone distribution, and fine visual resolution remained unexplored.

Using High-Resolution Imaging and Micro-Psychophysics

Harmening’s research team addressed this gap by employing an adaptive optics scanning light ophthalmoscope (AOSLO)—the only one of its kind in Germany. This tool allowed for precise examination of the connection between foveal cone density and the smallest details perceivable. While using the AOSLO, researchers also tracked eye movements as they measured the visual acuity of 16 healthy participants engaged in a challenging visual task. The AOSLO recorded the movement of visual stimuli across participants’ retinas, enabling identification of specific photoreceptors contributing to vision.

First author Jenny Witten from the Department of Ophthalmology at UKB and a PhD candidate at the University of Bonn analyzed these movements during a letter discrimination task.²

Eye Movements are Fine-Tuned to Cone Density

The study showed that humans can perceive finer details than the cone density in the fovea alone would suggest.

“From this, we conclude that the spatial arrangement of foveal cones only partially predicts resolution acuity,” Harmening noted.

Furthermore, the researchers found that eye movements enhance sharp vision; during fixation, drift movements synchronize with the fovea’s structure, continuously positioning the retina where cone density is greatest.¹

“The drift movements repeatedly brought visual stimuli into the region where cone density was highest,” explained Witten.

Within just a few hundred milliseconds, drift behavior aligned with higher-density retinal areas, enhancing visual clarity. Both the length and direction of these drifts were critical.¹

According to Harmening and his team, these insights deepen our understanding of the physiological basis of sharp vision.

“Recognizing how eye movements are optimized for clarity helps us better understand ophthalmologic and neuropsychological conditions and can guide improvements in technology that mimic or restore human vision, such as retinal implants,” he said.

This study received support from the German Research Foundation’s Emmy Noether Program, the Carl Zeiss Foundation (HC-AOSLO), Novartis Pharma GmbH (EYENovative research award), and the University of Bonn’s Open Access Publication Fund.²

Reference

1. Witten JL, Lukyanova V, Harmening WM. Sub-cone visual resolution by active, adaptive sampling in the human foveolar. eLife. 2024; DOI:10.7554/eLife.98648.3. Published October 29, 2024. Accessed November 2, 2024.

2. Subtle eye movements optimize vision. EurekAlert! Published October 29, 2024. Accessed November 4, 2024. https://www.eurekalert.org/news-releases/1062888