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Shedding Light on Night Vision
The retina of vertebrate eyes, including humans', is the most powerful light detector that we know. Researchers at the University's Center for Photochemical Sciences have created a highly sophisticated computer model of specialized sensors in the retina that has provided the first explanation for why, in very low light conditions such as dusk, light tends to be perceived as blue.
The group's findings were reported in the Sept. 7 edition of Science Magazine. The answer, they learned, lies in quantum mechanical effects that may one day shed light on currently incurable diseases such as so-called night blindness and be used to design the "ultimate sub-nanoscale light detector." It also opens new avenues of research into the evolution of vision in vertebrates.
"Night vision represents the last frontier of light detection," said Dr. Massimo Olivucci, whose research group in the Laboratory for Computational Photochemistry and Photobiology (LCPP) is engaged in developing new light-responsive materials based on concepts borrowed from nature.
"In extremely poor illumination conditions, such as those of a star-studded night or of ocean depths, the retina is able to perceive intensities corresponding to only a few photons, the indivisible units of light. Such high sensitivity is due to specialized sensors called rod rhodopsins that appeared more than 250 million years ago on the retinas of vertebrate animals," Olivucci said.
Both cone and rod rhodopsins are the interface between the physical world responsible for light detection and the physiological world of brain sensing. Cones are employed in daylight (color) vision, while rods are employed in night vision. All rhodopsins are proteins containing a derivative of vitamin A that serves as an "antenna" for photon detection. When a rhodopsin detects a photon, it becomes chemically activated and sends a message to the brain.
However, the rod rhodopsins can also be activated in cases of high body heat. Though extremely rare, the possibility of this "dark noise" is significant "because there are more than 2 billion rhodopsin molecules in a single rod cell and millions of such cells in the human retina," said LCPP graduate student Samer Gozem, a primary researcher on the project.
"A low level of 'dark noise' is necessary for the evolution of night vision" because if the rods are activated by ambient body heat instead of the presence of light, dim-light vision would be disrupted, Olivucci explained. The rhodopsin models have provided, for the first time, an understanding of the thermal activation process. Using the models, the researchers have been able to formulate a theory of thermal noise.
Blue light has a wavelength of about 470 nanometers, which the computer models have shown corresponds to the optimal (thus lowest) level of heat activation for perception of low light.
The team built rhodopsin models on a computer cluster at the Ohio Supercomputer Center, which has provided two independent research grants to the group. Working with French collaborator Dr. Nicolas Ferré at the Université d'Aix-Marseille, Gozem and research assistant Dr. Igor Schapiro carried out a series of state-of-the art simulations of the thermal activation of a set of rod rhodopsins.
The model construction, validation and study have taken more than two years to complete and were jointly funded by the Center for Photochemical Sciences and the College of Arts and Sciences.