Seeing better by looking away
A study suggests that we fixate slightly away from the retinal optimum to
see better

Date:
August 2, 2021
Source:
University of Bonn
Summary:
When we fixate an object, its image does not appear at the
place where photoreceptors are packed most densely. Instead,
its position is shifted slightly nasally and upwards from the
cellular peak. Researchers observed such offsets in both eyes of
20 healthy subjects, and speculate that the underlying fixation
behavior improves overall vision.



FULL STORY ==========================================================================
When we fixate an object, its image does not appear at the place where photoreceptors are packed most densely. Instead, its position is shifted slightly nasally and upwards from the cellular peak. This is shown in a
recent study conducted at the University of Bonn (Germany), published
in the journal Current Biology. The researchers observed such offsets
in both eyes of 20 healthy subjects, and speculate that the underlying
fixation behavior improves overall vision.


==========================================================================
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
many millions of photosensitive cells distributed evenly over the
sensor surface. These pixels all have the same size and are uniformly
packed. In the human retina, on the other hand, there are two types of
pixels: rod and cone photoreceptors. While rods help seeing in dim light,
cones enable us to see color and fine details.

In contrast to their technical counterparts, cones vary greatly in
size and spacing. In the fovea, a specialized central area conveying
sharpest vision, there are up to 200,000 cones per square millimeter;
this number drops to about 5,000 in the periphery. 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 at the Department of Ophthalmology at the University Hospital
Bonn 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 on that spot -- that, at least,
was the general assumption so far." Gaze is optimized to seeing with two
eyes As part of her doctoral thesis, Harmening's colleague, Jenny Lore'n Reiniger found that this is not quite the case. Her research showed that fixated objects are somewhat shifted nasally and upwards in relation to
the location of highest cone density -- in a systematic way. "We studied
20 subjects, and found this trend in all of them," Reiniger says. "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." At first glance, this seems paradoxical: Why do we not use
the sharpest part of our retina for seeing? Perhaps this 'trick' allows
us to reserve our eyes' maximum resolution for areas of the image that
need it most. "When we look at horizontal surfaces, such as the floor,
objects above fixation are farther away," Reiniger explains. "This is
true for most parts of our natural surrounds. Objects located higher
appear a little smaller. Shifting our gaze in that fashion might enlarge
the area of the visual field that is sheen sharply." The researchers
speculate that this behavior is an adaptation to seeing with two eyes.

The observed image shift is quite small. "The fact that we were able to
detect it at all is based on technical and methodological advances of
the last two decades," says Harmening. The researchers from Bonn use a laser-based adaptive optics ophthalmoscope, enabling them to directly see individual cones in the eyes of their participants. "The method also shows
us exactly which cells were used to fixate an object," says Harmening, who
is also a member of the transdisciplinary research area "Life and Health"
at the University of Bonn, and part of the Medical Imaging Center Bonn.

In the human retina, there are up to seven million of these tiny color receptors, but when we focus on a point we use only a small fraction of
them - - probably only a few dozen -- and possibly always the same ones throughout our lives. Cell targeted vision testing will likely help to understand which ones are the most important, in health and disease.

========================================================================== Story Source: Materials provided by University_of_Bonn. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Jenny L. Reiniger, Niklas Domdei, Frank G. Holz, Wolf
M. Harmening. Human
gaze is systematically offset from the center of cone
topography. Current Biology, 2021; DOI: 10.1016/j.cub.2021.07.005 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210802160638.htm

--- up 12 weeks, 3 days, 22 hours, 45 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)