We like to think of the eye as a camera, but the analogy falls short if we look at the distribution of light-sensitive cells – photoreceptors – in the human retina. In digital cameras, the sensor consists of several million photosensitive cells distributed evenly over the surface of the sensor. These pixels are all the same size and are evenly packed. In the human retina, on the other hand, there are two types of pixels: rod and cone photoreceptors. While the rods help see in dim light, the cones allow us to see colors and fine detail. Unlike their technical counterparts, cones vary widely in size and spacing. In the fovea, a specialized central area conveying the sharpest vision, there are up to 200,000 cones per square millimeter; that number drops to around 5,000 in the outskirts. This is comparable to a camera with varying resolution across the field of view, resulting in sharp and less sharp areas in the final image.

Dr Wolf Harmening, Head of the Adaptive Optics and Visual Psychophysics Group in the Ophthalmology Department at Bonn University Hospital, explains: “In humans, cone packing varies within the fovea itself. , with a sharp peak in its center. When we focus on an object, we align our eyes so that its image falls exactly where it is – at least that has been the general assumption so far. “

The gaze is optimized to see with two eyes

As part of her doctoral thesis, Harmening’s colleague Jenny Lorén Reiniger found that this is not quite the case. His research has shown that fixed objects move somewhat nasally and upward from the location of the highest cone density – consistently. “We studied 20 subjects and found this trend in all of them,” Reiniger explains. “The offsets were a bit larger for some and smaller for others, but the direction was still the same, and symmetrically between the two eyes. We also found that same spot when we repeated the measurement a year later. . “

At first glance, it seems paradoxical: why don’t we use the sharper part of our retina to see? Perhaps this “trick” allows us to reserve the maximum resolution of our eyes for the areas of the image that need it most. “When we look at horizontal surfaces, such as the floor, the objects above the fixture are further away,” Reiniger explains. “This is true for most parts of our natural environment. Objects higher up appear a bit smaller. Moving our gaze in this way could widen the area of ​​the visual field that is highly bright. The researchers believe this behavior is an adaptation to two-eyed vision.

The observed image shift is quite low. “The fact that we were able to detect it is based on the technical and methodological advances of the past two decades,” explains Harmening. The Bonn researchers use an adaptive optics laser ophthalmoscope, allowing them to directly see individual cones in the eyes of their participants. “The method also shows us exactly which cells were used to fix an object,” explains Harmening, who is also a member of the transdisciplinary “Life and Health” research area at the University of Bonn and the Medical Imaging Center Bonn.

In the human retina, there are as many as seven million of these tiny color receptors, but when we focus on one point, we only use a small fraction – probably only a few dozen – and maybe always. the same throughout our lives. Vision tests targeted to cells will likely help to understand which ones are most important, in terms of health and disease.

Reference: Reiniger JL, Domdei N, Holz FG, Harmening WM. The human gaze is systematically shifted from the center of the topography of the cone. bioRxiv. 2021.03.19.436115. doi: 10.1101 / 2021.03.19.436115

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