Muller cells-special light-guiding cells in the retina-split light and focus different wavelengths onto rods and cones, reports a paper this week in Nature Communications. This may improve daytime vision without negatively affecting night vision.

The human retina is arranged backwards, with the light sensing photoreceptor cells buried beneath layers of other cells as the last cells in the light path, rather than the first. Retinal Muller cells act as light guides, transferring light across the retina towards the photoreceptors but how this affects vision was not clear.

Perlman and colleagues used computer modelling, backed up by observations in the retina, to show that Muller cells act as funnels to focus red-green light onto cones, the light sensing cells most important during the daytime. At the same time they permit blue-purple light, the most common night time colours, to spill out onto surrounding rods.

In this way our eyes can gather daylight and focus it onto the cones that are able to detect it. At night, when light levels can be low and rods are more important, they get as much blue light as possible since the Muller cells simply let the light pass through.