Mice with enhanced colour vision were able to pick between coloured panels that normal mice could not distinguish

Credit: Gerald Jacobs

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CHICAGO: Genetically engineered mice, with human light receptor cells in addition to their own, have been shown to have enhanced colour vision.

The research may provide clues as to how our earliest primate ancestors acquired full-colour vision millions of years ago, and demonstrates the adapatability of mammalian brains, according to a study published today in the U.S. journal Science.

"If you gave mice a new sensory input at the front end, could their brains learn to make use of the extra data at the back end?" said study author molecular biologist Jeremy Nathans of the Johns Hopkins School of Medicine in Baltimore, Maryland.

"The answer is - remarkably - yes. They did not [require] additional generations to evolve new sight."

Under normal circumstances, mice and most other mammals, have 'dichromatic' vision. They typically view the world with a limited colour palette, which in human terms means yellows, blues and greys.

In contrast, humans and other primates have three types of light receptors in their eyes, fine-tuned to wavelengths that appear blue, green and red. The receptors give us 'trichromatic' vision, and allowing us to distinguish five colours.

Despite the fact that mice did not evolve to handle this more complex colour spectrum, the researcher found that by engineering them with a gene for an additional human light sensor, they were able to see in full colour.

Though Nathan's team knew the new transgenic mice were expressing the gene for the human light receptor, they did not initially know if they were able to 'rewire' their brains to use the new visual information.

To test this, they set up an experiment to determine if the mice could distinguish between different coloured light panels, designed to appear the same to normal mice. Mice set before them were trained to touch the one that appeared to differ from the other two. A correct answer was rewarded with a drop of soya milk.

Through tests involving 10,000 repetitions, the researchers found that the genetically engineered mice chose the correct panel 80 per cent of the time. In the same tests, a control group of normal mice, lacking the human light receptors, chose correctly just a third of the time.

The mammalian brain possesses a flexibility that permits a nearly instantaneous upgrade in the complexity of colour vision, wrote the researchers.

"It's been unclear [until now], whether the simple addition of a photopigment is sufficient to yield a new dimension of color vision - or whether you might need some changes in the nervous system too," said study co-author Gerald Jacobs, a psychologist at the University of California at Santa Barbara.

And the experiment may also mimic how our early primate ancestors acquired trichromatic vision - via mutations that led to a third photoreceptor registering longer red wavelengths of light, Nathans said. Experts think trichromatic vision evolved at least 40 million years ago in primates, and was down to a change that created a variant of a receptor gene located on the X-chromosome.

"What we are looking at in these mice is the same evolutionary event that happened in one of the distant ancestors of all primates and that led ultimately to the trichromatic colour vision that we now enjoy," said Nathans.

Prior research from England's University of Cambridge has suggested that trichromatic vision may have offered an instant survival advantage, as it allowed primates to distinguish between green, unripe fruits and red and orange-coloured ripe fruits.

More information:

Emergence of Novel Color Vision in Mice Engineered to Express a Human Cone Photopigment, Science