why the dimly lit night sky has a bluish cast

The swirling skies of Vincent Van Gogh’s Starry Night illustrate a mystery that has eluded biologists for more than a century–why do we perceive the color blue in the dimly lit night sky? A newly discovered mechanism of color vision in mice might help answer this question, Caltech researchers say.

The work, which was done in the laboratory of Markus Meister, Anne P. and Benjamin F. Biaggini Professor of Biological Sciences, will be published on April 14 in the print edition of the journal Nature.

In humans, vision is enabled by two types of light-sensitive photoreceptor cells called rods and cones. When these photoreceptors detect light, they send a signal to specialized neurons in the retina called retinal ganglion cells, or RGCs, which then transmit visual information to the brain by firing electrical pulses along the optic nerve.

A standard biology textbook would likely explain that vision in dim light is enabled by rods–sensitive light detectors that are only capable of producing black and white vision. Color vision, on the other hand, is enabled by cones, which are active in bright light. Humans have three types of cones, and each cone contains a different light-sensitive chemical, or pigment, that reacts to different colors, or wavelengths, of light. We have red-, green-, and blue-sensitive cones, and the brain perceives color by comparing the different signals it receives from nearby cones of each type.

To explore whether or not there were other modes of color vision, Meister and his team studied another mammal: the mouse. Previous behavioral studies indicated that mice are indeed capable of some form of color vision. As in humans, that vision is dependent on light signals picked up by cones. Mice have two types of cones–one that is sensitive to medium-wavelength green light and one that is sensitive to short-wavelength ultraviolet light (UV).

“The odd thing about the mouse is that these two kinds of cones are actually located in different parts of the retina,” Meister says. “Mice look at the upper part of the visual field with their UV cones and the lower part with their green cones. We wanted to know how a mouse perceives color when any given part of the image is analyzed with only one cone or the other cone–meaning the brain can’t compare the two cone signals to determine a color.”

Source: https://www.sciencedaily.com/releases/2016/04/160414145217.htm