Not blinded by the light: Rods in the retina contribute to daylight vision

The photoreceptors in the retina, at the back of the eyes, are the primary light sensitive cells that allow us to see: they convert light into electrical signals. There are two types, rods and cones, and it has generally been assumed that rods are responsible for seeing in dim light conditions, whereas cones allow vision in bright light. This division of labor between rods and cones can be found in virtually every biological and medical text book.

A new study challenges this traditional view: A group of researchers from the universities of Tübingen, Manchester and Helsinki led by Thomas Münch from Tübingen shows that rod photoreceptors do, in fact, contribute to daylight vision. Most surprisingly, their contribution even increases when the daylight becomes brighter, up to the brightest light levels that would ever be encountered in natural environs.

Using transgenic mice without functional cones, the investigators first measured rod-driven signals and could reliably detect them both in the retina and in the brain even at high light levels. Furthermore, they were subsequently able to show this bright-light rod contribution in mice with fully functional cones as well.

With this new data, it seemed obvious that the models used by most scientists beforehand must be incomplete. And indeed, much was already known about rod physiology that had not been included in the big picture, as it has been understood up to now. Adding this information, the research team arrived at a model that not only explains how rods see so well in dim light, but also their own findings: that rods continue to function in bright light.

"We showed that rods can function at bright light," says Thomas Münch, "but it is true that cones can do this much better and much more reliably. However, these new insights may still open new avenues towards treatments for patients without functional cones, so-called rod monochromats." People in modern times are exposed to artificial bright light for many hours each day. In the old paradigm, this has made reliance on rods for the development of treatments counterintuitive. With these new mechanistic insights into bright rod vision, however, there may be new possibilities for therapies for patients without functional cone vision.