University of Manchester scientists have found that a new type of light sensitive cell (photoreceptor) in the eye helps the brain measure brightness. This mechanism, which works alongside the rod and cone photoreceptors, may be important for people with certain types of blindness.

Professor Rob Lucas and Dr. Tim Brown, whose results will be published next week in the online, open access journal PLoS Biology, hope their findings will lead to a new understanding of visual perception, and may eventually even lead to technical applications in artificial lighting and visual displays.

When activated by light, rod and cone cells in the eyes send information about movement, shape and color to the brain through a pathway called the optic nerve. In this new study, Dr. Lucas' team has found that a small percentage of retinal neurons that produce a light sensitive protein known as melanopsin also play an integral role in measuring the brightness of the world around us. It was thought that these melanopsin cells detected light primarily for subconscious responses to light, such as regulating pupil size. However, the team found that melanopsin cells also transmitted functional information about brightness to brain regions involved in conscious perception. Importantly, this occurred in both sighted mice and in mice previously considered blind.

Dr. Brown, at Manchester's Faculty of Life Sciences, said, "This reveals a new role for melanopsin in perceiving brightness and suggests that these cells could support vision even in people with advanced retinal degeneration. Loss of rods and cones is a common cause of blindness, but people suffering these conditions often retain some light perception. The accepted explanation for such cases has been that they reflect incomplete rod and cone loss. We have shown that in mice with similar conditions light responses can in fact rely on melanopsin."

Professor Lucas added, "Now we are asking to what extent melanopsin could help the normally sighted to see, and what it might contribute to the blind and partially sighted. So far, our results suggest that we rely upon melanopsin to perceive the difference in brightness between a foggy day in London and bright summer sunshine in Manchester. We need to know whether it also provides the spatial information we rely upon for more complex sorts of vision. Our findings could change the way that we think about artificial lighting and may even improve the ability of computer displays or televisions to reproduce our perception of the real world. At the moment both of these technologies are designed only with rod and cone cells in mind."

The study, in which Professor Lucas and Dr. Brown collaborated with researchers at the Salk Institute in San Diego and University College London's Institute of Opthalmology, was funded by the Wellcome Trust.

Funding: This work was funded by grants from the Wellcome Trust (RJL and HDP), National Institutes of Health grant EY016807, Dana Foundation grant and Pew Scholar award to SP, JSPS Fellowship to MH, and was supported in part by the London Project to Cure Blindness. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests statement: The authors declare that no competing interests exist.

Citation: Brown TM, Gias C, Hatori M, Keding SR, Semo M, et al. (2010) Melanopsin Contributions to Irradiance Coding in the Thalamo-Cortical Visual System. PLoS Biol 8(12): e1000558. doi:10.1371/journal.pbio.1000558

Source:
PLoS Biology