Mice learn to see in colour

Mice have had a human gene introduced into their eyes to give them colour vision similar to that enjoyed by people.

The experiment provides new hope to blind people that the brain can adapt more quickly than thought to new sensory information, for instance from an artificial retina of the kind currently under development in various labs.

And it sheds new light on the evolution of colour vision, a topic of intensive study for more than three decades, showing that the ability to detect a broader spectrum of light had knock on effects on brain development.

The light-sensing retina in the eyes of primates such as humans and monkeys is unique among mammals in that it has three pigments that absorb short (blue), medium (green) and long (red) wavelengths of light.

Mice, like other mammals, only have two pigments; one for short and one for medium wavelengths.

As a result, mice have dichromatic vision (which, in human terms, registers yellows, blues and greys), similar to what some people with red-green colour blindness see.

Although mice, like most mammals, typically view the world with a limited colour palette scientists have now transformed their vision by introducing a single human gene into a mouse chromosome.

The human gene codes for a light sensor that mice do not normally possess, and its insertion allowed the mice to distinguish colours as never before.

For the new study in the journal Sciencea team at Johns Hopkins University in Baltimore, Maryland, together with researchers at the University of California at Santa Barbara, introduced a human long wavelength receptor on the X chromosome, which made the mice produce a third pigment that registered longer wavelengths of light.

The human genes were biologically functional in the mice, but the real question was whether the mice could use the new visual information.

“It’s been unclear,” Prof Gerald Jacobs explained, “whether the simple addition of a photopigment is sufficient to yield a new dimension of colour vision, or whether you might need, in addition, some changes in the nervous system.”

The new abilities of the genetically engineered mice indicate that the brain does indeed possess a flexibility that permits a nearly instantaneous upgrade in the complexity of colour vision, say the study’s senior authors, Prof Jacobs and Jeremy Nathans.

When the team tested the mice on tasks involving distinguishing differently coloured panels and lights, the results indicated that the mice had new colour vision, revealing how genetic mutations may have ultimately allowed our ancestors to see five colours in a rainbow.

Once genetic changes produced additional photopigments in primates, which is thought to have occurred more than 40 million years ago, the animals’ brains were able to “rewire” themselves in order to process the new signals and allow full-colour vision.

“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?” asks Prof Nathans. “The answer is, remarkably, yes.

They did not require additional generations to evolve new sight. 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.”

There are several theories to explain why colour vision helped our ancestors.

John Mollon at the University of Cambridge has suggested that it allowed primates to discriminate between unripe fruit, which is typically green, and ripe red- and orange-coloured fruits.

But a team from the California Institute of Technology, Pasadena, has challenged this idea with evidence that colour vision is crucial for seeing changes in the skin of others - whether they are red with rage, flushed with embarrassment or white with fear.