It's been long understood that our vision is attributed to two types of cells within our eyes, rods and cones. Each detect light in a different way, and have their own respective strengths and weaknesses. A new discovery is helping to shed light on the mechanism by which we see colour.
Research performed in the laboratory of Markus Meister, Anne P. and Benjamin F. Biaggini Professor of Biological Sciences has been published in the academic journal Nature, in April 2016. It's focus is on the two main functions of the light-sensitive cells within human eyes. Rods, which a very sensitive and work well in dim light, but don't sense colour very well, and cones, which require lots of light to function, and detect a wide range of colours. In fact there are three types of cone cells, each responsible for one of the three primary colours of light, red, green, and blue. Together, they can sense every colour of the rainbow. These cells send signals through the optic nerve and into the brain, where they are then interpreted as images.
The study aimed to discover a possible new form of colour vision which humans may be using, but that science is not yet aware of. By using laboratory mice as test subjects, researchers may be able to isolate how different photoreceptors cells work in detail.
Mice only have two types of cone cells, one that detects medium-wavelength green light and one that detects short-wavelength ultraviolet light.
"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 colour 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 colour."
Through experimentation, researchers learned that a specific type of neuron in the mouse retina, referred to as a J-RGC, was special, in the way it sent colour to the brain. These photoreceptors fire faster in response to green light, but stop firing in response to ultraviolet light. Interestingly, the J-RGCs were turned on by green light even in the upper part of the visual field, which contained no green cones.
Additional experiments were conducted, and Meister and his team were able to learn how the J-RGC compares signals from the ultraviolet cones to signals from rods, which are also sensitive in the green part of the spectrum. For the first time ever, it became apparent that an antagonistic relationship between rods and cones exist. Rods excite a neuron called a horizontal cell, which then inhibits the ultraviolet cones.
Curious how this colour vision system would be helpful to a mouse in its natural environment, Meister and his colleague, first author Maximilian Joesch from Harvard University modified a camera with filters that would replicate the wavelengths sensed by the mouse rods and cones. Th camera was used it to take images of plants and materials that a mouse would naturally encounter in the wild.
The images taken by he camera show that both seeds and mouse urine are highly visible in the green and ultraviolet areas of the colour spectrum. It's hypothesized that these two visual cues are helpful in spotting food sources, and communicating with other nearby mice, much in the same way dogs mark their territory by urinating on objects.
Meister believed there is a similar function within human eyes responsible for our perception of the colour blue in dim light. In the human retina, the horizontal cell preferentially inhibits the red and green cones, but not the blue cones.
"In really dim light, our cones don't receive enough photons to work, but they continue to emit a low-level baseline signal to the rest of the retina that is independent of light," Meister explains.
"The rods are active, however, and through the horizontal cell they inhibit both the red and green cones. Because this baseline signal from the red and green cones is suppressed, it looks like the blue cones are more active. To the rest of the retina, it seems like everything in the field of vision is blue."