At Technion’s Faculty of Physics, Dr Erez Ribak and master’s student Amichai Labin have solved a mystery that had puzzled optical scientists for 120 years. Their findings, published in April 2010 in the prestigious scientific journal Physical Review Letters, reveal that the retina is optimally designed to improve the sharpness of images resulting in enhanced human visual acuity.
A section through the retina, colored here
“Using optical analysis, it is possible for the first time to understand how the structure of the retina helps sharpen our view of the world,” explains
Ribak. “The retina is inverted - for a reason. Our eyes are built like a digital camera, with a lens in front and a detector - the retina - at the back.”
for demonstration purposes only. Light
arrives from the pupil (above) and is
captured in the funnels of the glial (Müller)
cells and concentrated down to the cones,
with the rest scattered to the rods. The
retina is one-quarter to one-half mm thick.
At the far side of the retina lie the photoreceptors (rods and cones), strangely covered with transparent layers of neurons. The neurons serve as wiring that process and pass the detected image to the brain, but also distort this image. While the eye’s structure and function have been known since the late 19th century, scientists were mystified why this wiring is not found behind the photoreceptor cells, and why this feature is common to all vertebrates. The first clue came three years ago when German scientists found that glial
(Müller) cells, which intersect the retina across the neural layers, are able to transmit light.
Ribak and Labin constructed an optical model of the retina, and passed light through this volume. They found that only light which came through the center of the pupil was captured in the glial cells and guided directly to the cone photoreceptors, responsible for color vision. Light leaking from neighboring cells or coming from the periphery, which would clutter our sight, was rejected and scattered away, to be picked up by the gray-sensitive rod photoreceptors. “This feat could not be achieved if the photoreceptors came before the neural layers,” Ribak concludes. The new understanding of the role of glial cells might find applications in more successful eye transplants and better camera designs, says
Another aspect of color imaging was solved by this model. “As the different colors form images at different depths of the retina, we should have seen only one color in focus,” Ribak explains. “For example, if we see the blue image sharply, the red image of the exact same scene would seem blurred to our eyes. However, because the glial cells collect and concentrate all colors into the same cone photoreceptors, we see all colors equally focused.”