Mammalian brain shows remarkable plasticity

Key Points

Surprisingly, however, it turns out that color vision in mammals has nothing to do with recognizing human hair color. Rather, according to an article in Scientific American, trichromacy (the presence of three visual pigments for color vision) is relatively rare in animals and confers a selective advantage over dichromats in the ability to see ripe fruit against a background of green foliage. I encourage everyone to read this fascinating story of color vision.

In trichromats, one pigment absorbs light maximally at wavelengths about 430 nm (blue), another at approximately 530 nm (green), and a third at approximately 560 nm (yellow). Using his prism, Sir Isaac Newton showed that white light can be broken into a rainbow of colors, then reassembled into what we perceive as white. Our brains perceive colors related to the varying degrees of stimulation of our three types of pigment-containing cones.

Mice are dichromats (presumably it is not very important for these little guys to determine the color of the fur of the cat chasing them). The article describes an amazing experiment by one of the authors in which mice were genetically engineered to express the pigment that absorbs at 560 nm in human retinas. Not only did this pigment appear in some of the cone cells of these mice, but, amazingly to me, the mice immediately could use that enhanced color vision to discriminate among colors indistinguishable to other mice.

The electrical impulses generated in the cones must pass through the retinal interneurons to the ganglion cells, traverse ganglion cell axons to reach the occipital cortex, and then pass to the parietal association cortex. Given that these mouse brains never have processed the signals generated by three visual pigments, it seems an amazing testament to the brain that it could so readily adapt to this new input. The authors state "the experiment demonstrates the remarkable plasticity of the mammalian brain, because the mouse can use its new pigment without having nerve cells specifically wired for interpreting its signals . . . the mammalian brain has the ability to extract information from novel and qualitatively different types of visual input."

In this age of gene therapy, one easily can imagine eyes with cones manipulated to express pigments sensitive to wavelengths invisible to the rest of us. What about an infrared-sensitive pigment, allowing an animal literally to see in the dark?

The article implies that nature already might be experimenting along these lines. Because of a mutation in one of the pigment genes, some women have four types of cone visual pigments. Whether this fourth pigment significantly alters color perception in these women is an unanswered question.

But on one point no doubt exists: as a general rule, people with blue eyes are irresistible to members of the opposite sex.

Peter J. McDonnell, MD director of the Wilmer Eye Institute, Johns Hopkins University School of Medicine, Baltimore, and chief medical editor of Ophthalmology Times.

He can be reached at 727 Maumenee Building, 600 N. Wolfe St., Baltimore, MD 21287-9278 Phone: 443/287-1511 Fax: 443/287-1514 E-mail: pmcdonn1@jhmi.edu

Reference

1. Jacobs GH, Nathans J. The evolution of primate color vision. Sci Am. 2009 Apr;300:56-63.