Helping humans see in bright and low light

Ever wonder how your eyes adjust during a blackout? When we go from light to near total darkness, cells in the retina must quickly adjust. Vision scientists at Washington University School of Medicine in St. Louis have identified an intricate process that allows the human eye to adapt to darkness very quickly. The same process also allows the eye to function in bright light.

The discovery could contribute to better understanding of human diseases that affect the retina, including age-related macular degeneration, the leading cause of blindness in Americans over 50. That's because the disease and the pathway the researchers have identified both involve cells called cone cells.

Age-related macular degeneration may be modulated, perhaps, through this pathway we've identified in the retina, says principal investigator Vladimir J. Kefalov, Ph.D. Deficiencies in this pathway affect cone cells, and so does macular degeneration, so it's possible that if we could enhance activity in this pathway, we could prevent or reverse some of that damage to cone cells.

The retina's main light-sensing cells are called rods and cones. Both use similar mechanisms to convert light into vision, but they function differently. Rods are highly sensitive and work well in dim light, but they can quickly become saturated with light and stop responding. They don't sense colour either, which is why we rarely see colours in dim light. Cones, on the other hand, allow us to see colours and can adapt quickly to stark changes in light intensity.

The researchers began with studies of salamanders because their cone cells are abundant and easy to identify. Cones rely on light-sensing molecules that bind together to make up visual pigments. The pigments get destroyed when they absorb light and must be rebuilt, or recycled, for the cone cells to continue sensing light. After exposure to light, key components of pigments called chromophores can leave the cells and travel to the nearby pigment epithelium near the retina. There the chromophore is restored and returned to the photoreceptor cells.

Earlier this year, the research team removed the pigment epithelium layer in salamander retinas, so that pigment molecules could not be recycled that way. Then they exposed retinal cells both to bright light and to darkness. The rods no longer worked, but the cones continued to function properly, even without the eye's pigment epithelium.

Exposure to bright light destroyed visual pigments in rods, and those cells could not recycle chromophores, says principal investigator Vladimir J. Kefalov, Ph.D., assistant professor of ophthalmology and visual sciences. Pigments in cones, by contrast, quickly regenerated and continued to detect light even without the pigment epithelium, so it was clear a second pathway was involved.

In the new study, Kefalov did the same experiments in cells from mice, primates and humans with the same result.

To learn how cones were able to recycle pigments without pigment epithelium, Kefalov's team has focused on a particular type of cell in the retina. Called Müller cells, these cells support and interact with rods and cones. The researchers treated mouse retinas with a chemical that destroyed the Müller cells, then exposed the retina to bright light, followed by darkness.

When we blocked the function of Müller cells, the retinal visual pathway could not function because cones ran out of photo pigment and could not adapt to dark, Kefalov says.

The new paper, published in the journal Current Biology, suggests Müller cells are key to this pathway in mammals, including humans.

When those cells function properly, cones in the mouse, primate and human retinas are able to function in bright light and adapt to darkness, independently of the pigment epithelium, Kefalov says.

He says this discovery means it may one day be possible to manipulate this pathway in the retina to improve vision when the other pathway, involving pigment epithelium, has been interrupted by injury or disease, such as age-related macular degeneration.

Light at the speed of a bicycle

Photo: Ibrahim Iujaz/Wikimedia CommonsThe speed of light, 300 million metres per second was long thought an immutable constant and has defined our understanding of matter and energy but recent research in the area of optics and photonics is proving that we can manipulate light to some ingenious and hugely lucrative ends.

From the use of adaptive optics to catch perfect images of distant galaxies or detailed representations inside bio-specimens of, for example, mouse embryos, to electromagnetically induced transparency which can slow light to the speed of a bicycle - the field of optics and photonics is in the scientific vanguard.

The Institute of Physics (IOP) and the Engineering and Physical Sciences Research Council (EPSRC) are launching a new report entitled Optics and photonics: Physics enhancing our lives, to highlight the most recent advances in the field and demonstrate the potentially lucrative ends a range of researchers have in sight.

This field of research has already had a revolutionary effect on all of our everyday lives. From optical fibres, hair-like strands of silica glass that transmit vast amounts of data around the world as fleeting pulses of light, to lasers built in to our CD and DVD players, much of the developed world's technological infrastructure is already structured on physicists' ability to play with light.

There is however still much more to come. Just one of the research areas highlighted in the report, plasmonics, shows how our understanding of light, in this case stemming from the way light interacts with silver and gold to give off a shiny sparkle, could lead to the development of metamaterial-based invisibility cloaks or super-strength solar cells for renewable energy generation.

Advances will have dramatic effects on technology used in security as, for example, iris recognition systems become more sophisticated; communications will become swifter and securer; medical imaging techniques will become more precise; and we will be able to manipulate the plentiful energy of the Sun to greater human benefit.

Dr Robert Kirby-Harris, chief executive at the Institute of Physics, said, "We hope that this booklet illustrates how the research investment made by the EPSRC, and the support provided by the Institute to its membership, will enable the UK economy, and society at large, to benefit from the discoveries and advances being made in leading-edge physics research."