Topographic Offset of Human Gaze from the Fovea

Article: Seeing better by looking away
Source: University of Bonn (Germany), via ScienceDaily
Published: August 2, 2021

Preferred retinal locus of fixation
on two different days, one year apart

Unlike the photosensitive pixels of a camera, with which the human retina is often analogized, the photoreceptors of the eye, rods and cones, vary in size and spacing. Despite the large variability in foveal topography between individuals, however, there is surprising precision with which fixation is repeatedly directed to a small cluster of cones in the foveola, according to a new study. As part of a doctoral thesis, the study suggests that when we fixate an object, its image is shifted slightly nasally and upwards from the cellular peak at the fovea, contrary to the general assumption. This finding was observed in both eyes of 20 study participants and was made possible by technological and methodological advances over the past two decades, since the observed image shifts were very small. On average about 5 minutes of arc of visual angle, the shift is mirrored between the two eyes. Specifically, the researchers used a laser-based adaptive optics ophthalmoscope to directly see the individual cones in the eyes of the participants and see exactly which cells were used to fixate an object. According to the study's first author, "The offsets were a little larger for some and smaller for others; yet the direction was always the same, and symmetrically so between the two eyes. We also found that same spot when we repeated the measurement a year later." She speculates that the paradox is an adaptation for binocular vision, explaining, "When we look at horizontal surfaces, such as the floor, objects above fixation are farther away...Objects located higher appear a little smaller. Shifting our gaze in that fashion might enlarge the area of the visual field that is seen sharply." The researchers think the functional link between the cellular mosaic of the fovea and visual behavior can help to develop cell-targeted vision testing to better understand health and disease.

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Reiniger JL, Domdei N, Holz FG, et al. "Human gaze is systematically offset from the center of cone topography." Current Biology.  2 August 2021. https://doi.org/10.1016/j.cub.2021.07.005