New Light Shed On How Retina's Hardware Is Used in Color Vision

Biologists at New York University and the University of   Würzburg have identified, in greater detail, how the Retina's cellular   hardware is used in color preference. The findings, published in the   latest issue of the Proceedings of the National Academy of Sciences (PNAS), enhance our understanding of how eyes and the brain process color.

Light can serve as an attractive or repulsive landmark for   orientation -- we identify an object or a light source at a certain   location in visual space, then approach it or retreat from it. This   process, called phototaxis, was the focus of the PNAS study.

Conducted by biologists at New York University's Center for   Developmental Genetics and the Department of Genetics and Neurobiology   at the University of Würzburg in Germany, the research specifically   examined the photoreceptor cells in the Retinas of the fruit fly Drosophila. Drosophila is a powerful model for studying the color vision process as it is   amenable to very specific genetic manipulations, allowing researchers to   analyze how its visual system functions when different elements of its   Retina are affected.

The visual systems of most species contain photoreceptors with   distinct spectral sensitivities that allow animals to distinguish lights   by their spectral composition (i.e., color). In Drosophila,   six of these (R1-R6) are responsible for motion detection and are   sensitive to the brightness or dimness of a broad spectrum of light. Two   others (R7 and R8) are used for color vision by comparing ultraviolet   light (UV), detected by R7, with green or blue light detected by two   types of R8. The NYU and University of Würzburg biologists investigated   how photoreceptor types contribute to phototaxis by blocking the   function of either R7 or R8, or a combination of a range of   photoreceptors (R1-R6, R7 and/or R8).

In the study, they constructed two sets of "Y-shaped mazes" with two   different types of light at the ends of each: UV and blue in one and   blue and green in the other. Under this arrangement, the fly would show a   preference for certain type of light (UV vs. blue in one maze; blue vs.   green in the other) by moving toward it. The researchers could then   link specific preferences to the make-up of each fly's visual system.

In a "UV vs. blue" choice, flies with only R1-R6 and flies with only   R7/R8 photoreceptors preferred the blue to the UV light. This finding   suggested that these two sets of photoreceptors (R1-R6 and R7/R8)   function separately in phototaxis as flies with only one of these sets   showed similar preferences. In addition, flies without a functioning R7   photoreceptor preferred the blue to the UV light, whereas flies without   R8 preferred UV. In the "blue vs. green" maze, flies without a   functioning blue R8 photoreceptor preferred green, whereas those with a   defective for green R8 photoreceptor preferred blue. This shows that   each subclass of photoreceptors [R1-R6, R7, R8 (blue), R8 (green)] is   used by the fly to distinguish colors and setup its innate color   preference. In a previous work, the same authors had shown that motion   detection only involves R1-R6 and not R7 and R8, suggesting that there   are two independent channels in the fly visual system -- one for motion   and one for color.

"This simple insect can achieve sophisticated color discrimination   and detect a broader spectrum of colors than we can, especially in the   UV," said NYU biologist Claude Desplan, one of the study's authors. "It   is a great model system to understand how the Retina and the brain   process visual information.

The research was supported by a grant from the National Institutes of Health.

http://www.sciencedaily.com/releases/2010/03/100308151051.htm

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