Cue the super-mouse. Scientists have engineered mice that can see infrared light normally invisible to mammals — including humans.
To do so, they injected into the rodents’ eyes nanoparticles that convert infrared light into visible wavelengths1.
Humans and mice, like other mammals, cannot see infrared light, which has wavelengths slightly longer than red light — between 700 nanometres and 1 millimetre.
But Tian Xue, a neuroscientist at the University of Science and Technology of China in Hefei, and his colleagues developed nanoparticles that convert infrared wavelengths into visible light. The nanoparticles absorb photons at wavelengths of around 980 nanometres and emit them at shorter wavelengths, around 535 nanometres, corresponding to green light.
Xue’s team attached the nanoparticles to proteins that bind to photoreceptors — the cells in the eye that convert light into electrical impulses — and then injected them into mice.
The researchers showed that the nanoparticles successfully attached to the photoreceptors, which in turn responded to infrared light by producing electrical signals and activating the visual-processing areas of the brain.
The team conducted experiments to show that the mice did actually detect and respond to infrared light.
In one test, they gave mice the choice between a dark box and a box ‘illuminated’ with infrared light. Normally, mice — which are nocturnal — will seek out the safety of a darker box. The ordinary mice showed no preference between the two boxes because they couldn’t see the infrared light. But the modified mice favoured the dark box.
In another experiment, the team taught both types of mouse to associate green light with an electric shock, but the modified mice also froze in fear when an infrared light was turned on.
Finally, the researchers placed the rodents in a water maze that had two arms illuminated by different light patterns, only one of which led to a hidden, dry refuge. The modified mice chose the correct arm of the maze according to the light pattern, regardless of whether the patterns were displayed in visible or infrared light.
“It’s sometimes a little bit creepy,” says Xue. “You show different patterns to the mouse which you cannot see — to you, it’s just an empty screen. But the mouse can choose it correctly.”
Other groups have also sought to give rodents infrared vision. Eric Thomson, a neuroscientist at Duke University in Durham, North Carolina, developed a system that allowed rats to detect infrared light through four sensors connected directly to the brain2. But the small number of sensors only provided enough visual information for the rats to find the location of a light, says Thomson.
“What is really exciting here is that they actually showed that they got real image information,” he says.
Xue says that his technique could have several applications, including giving people “super-vision”. Seeing infrared light could help people to see at night, by detecting infrared wavelengths emitted by, or reflected off, people and objects in the environment. This could be useful for military and security operations, for example.
The team also hopes to adapt the nanoparticles to carry drugs for later release in the eye. But there are several hurdles, including safety concerns, before any use in humans can be tested.
For example, the team’s nanoparticles contained heavy metals and regulators would be unlikely to approve them for use in humans, Xue says, so the team is developing organic versions.
But not everyone thinks this technique could be used to augment human vision.
The human visual system has evolved over millions of years to be sensitive to a highly specific part of the electromagnetic system, says Glen Jeffery, a visual neuroscientist at University College London, and the retina is not used to seeing infrared. It’s uncertain how people would interpret the image: the environment would appear a lot brighter, for example, and the images could be overwhelming.
So although the science is technically impressive, says Jeffery, it’s unclear what impact the technique will have. Given his apprehensions, he adds, “I am the last person in the world who would want to see infrared.”