Light Propagation in the Retina

Colour sorting in the eye by optical fibres

The image of the world, falling on the retina at the bottom of the eye, passes thick layers of cells before reaching the photoreceptors (light detectors). One would have expected the order to be the other way round: first the photoreceptors, then the neural layers whose job is to analyse the image in order to pass it on to the brain. The reason for that was discovered recently: the retina has fibre optics leading the light directly into the cone photoreceptors, in charge of colour vision. Light leaking between the fibres or arriving from the side of our pupil and might thus reduce our acuity is scattered around to arrive at the rod photoreceptors, which are not colour-sensitive. Thus we were able to explain a mystery which puzzled scientists for over a century: without these thick layers it would not have been possible to lead the light in the fibres, and improve acuity.

The computer model constructed in this research to describe light transmission through the retina showed something surprising: most of the light concentrated in the fibres is green and red, and it is being passed on directly to the cones most sensitive in these wave-lengths, green and red. Arguments whether this concept is right are still going on, since it is not trivial to verify that the fibres exist and that indeed they pass the light as expected. To confirm the model, we transported white light through real retinas, and measured the transmission, colour by colour.

Indeed, in a three-dimensional microscopic image it became clear that the structure of the fibre optic cells (also known as glial or Mueller cells) passes exactly the colours matching the cones. The rest of the light is scattered around, and hits the rods. That is, during the day time most of the light is concentrated (up to ten times!) into the colour-sensitive cones, while at night it is spread to the rods, such that night vision is hardly affected. In the experimental figure we see the light arriving from above, crossing through the light guides, and hitting the photoreceptors at the bottom of the retina.

This mechanism exists in all vertebrates, which shows its importance: despite the development of many species, the retina has kept this structure of light guiding cells intact, in order to improve the quality of the colour image at day and its intensity at night.

This research was performed by graduate students Amichai Labin and Shadi Safuri, and researchers Ido Perlman and Erez Ribak, from the departments of physics and medicine, and appeared in Nature Communications. Another popular explanation of the work appears in The Scientist Magazine.

Experiments

1. Adaptive optics system for the eye
2. Ocular wave front sensing
3. Acousto-caustic modulation for removing speckle from the wave front sensor
4. High-resolution imaging of the retina, and immersion optics for reduction of corneal aberrations
5. Light propagation in the retina: glial cells sort colors onto cones and rods

Simulations

1. Ocular aberrations
2. Retinal aberrations (ongoing work) scientific draft, layman introduction, more explanations
   1. Construction of geometric model of neural layers, glial (Muller) cells in the parafovea, outside the central high-sensitivity macula
   2. Attachment of relevant refractive index to layers
   3. Usage of the split-step Fourier-transform beam propagation method
      1. Verification test on cones
      2. Good match to analytic results
   4.  Propagation of light through retinal layers
      1.  Incidence angles from zero to maximum permitted through pupil
      2. Wave lengths from blue to near infra red
   5. Results so far
      1. Rejection of background and clutter: scattered light from light paths or from other directions does not reach into cones, responsible for colour vision
      2. Rejection of aberrations: high modes (very tilted wave fronts, as a result of chromatic, other aberrations) are scattered off
      3. Scattered light which did not arrive in cones can be detected by intervening rods, responsible for high sensitivity (but colour blind)
      4. Good fit to experimental results by Franze et al. (2007) for glial cells
      5. Might explain why the retina is inverted. If cones came first and neural layers behind, then the previous results would not have been valid:
         1. Background light would have been too high
         2. Most of the light would have reached the rods, not the cones
         3. There would have been a higher sensitivity to aberrations in the optics of the eye
         4.   There was no colour separation green and red to the cones, blue and far red to the rods