It’s an impressive example of using technology to expand an animal’s “umwelt”—the thin sliver of reality that it’s capable of perceiving. But as far as infrared vision goes, it’s a bit of a cheat. The mice aren’t seeing in infrared; they’re seeing infrared information that’s been changed into a more perceptible form.
But “science has always worked to convert invisible information into the range we can perceive,” says David Eagleman, a neuroscientist from Stanford University. “This is what microscopes and telescopes do: changing the very small or very distant into a form we can digest with our eyes. But instead of building a large piece of equipment to do the conversion, these investigators engineered a microscopic solution, directly mating technology to biology.”
“I love this stuff,” he adds. “It’s wonderfully clever.”
The nanoparticles that Xue used were originally developed for a completely different reason. Fourteen years ago, researchers figured out a way of infusing neurons with light-sensitive molecules, allowing them to control these cells with flashes of light. This technique, known as optogenetics, paved the way for many powerful studies, allowing neuroscientists to precisely manipulate the brains of living animals. It also holds promise for treating several brain diseases.
But there’s a catch: Optogenetics relies on molecules that are triggered by blue or yellow light, both of which are blocked by skin and muscle. To deliver that light into an animal’s brain, researchers must use either invasive optic fibers or implanted LEDs. They could dispense with these cumbersome devices if they simply made optogenetics compatible with infrared light, which more easily penetrates through flesh.
Gang Han from the University of Massachusetts Medical School managed to do that by creating a range of nanoparticles that absorb infrared and emit visible light. And while talking to Xue, his colleague who studies mouse vision, Han wondered, “What would happen if we inject these into a mouse’s eye?”
First, Han bolstered his particles with a protein called ConA, which helps them adhere to the light-detecting cells of the retina. When injected, they formed an even and lasting layer over the cells. And when Xue’s team then shone an infrared beam into the rodent’s eyes, their pupils constricted—a subconscious reaction that clearly showed that they could see the light.
Next, the team put the mice into a pair of linked chambers—one dark and one bathed in infrared. Normal rodents can’t tell the difference between these spaces, and spend equal amounts of time in each. But to the altered individuals, the infrared chamber was bright and off-putting; they spent most of their time in the dark space instead.
Finally, the team plopped the mice into a flooded, Y-shaped arena. The two prongs of the Y displayed different patterns of light—say, a triangle or a circle—and one of these indicated the presence of a hidden platform upon which the mice could rest. If the patterns were in infrared, the altered rodents would choose the right prong, but their normal peers would not.