US researchers have effectively given laboratory rats a "sixth sense" using an implant in their brains.
An experimental device allowed the rats to "touch" infrared light - which is normally invisible to them.
The team at Duke University fitted the rats with an infrared detector wired up to microscopic electrodes that were implanted in the part of their brains that processes tactile information.
The results of the study were published in Nature Communications journal.
The researchers say that, in theory at least, a human with a damaged visual cortex might be able to regain sight through a device implanted in another part of the brain.
Lead author Miguel Nicolelis said this was the first time a brain-machine interface has augmented a sense in adult animals.
The experiment also shows that a new sensory input can be interpreted by a region of the brain that normally does something else (without having to "hijack" the function of that brain region).
"We could create devices sensitive to any physical energy," said Prof Nicolelis, from the Duke University Medical Center in Durham, North Carolina.
"It could be magnetic fields, radio waves, or ultrasound. We chose infrared initially because it didn't interfere with our electrophysiological recordings."
His colleague Eric Thomson commented: "The philosophy of the field of brain-machine interfaces has until now been to attempt to restore a motor function lost to lesion or damage of the central nervous system.
"This is the first paper in which a neuroprosthetic device was used to augment function - literally enabling a normal animal to acquire a sixth sense."
In their experiments, the researchers used a test chamber with three light sources that could be switched on randomly.
They taught the rats to choose the active light source by poking their noses into a port to receive a sip of water as a reward. They then implanted the microelectrodes, each about a tenth the diameter of a human hair, into the animals' brains. These electrodes were attached to the infrared detectors.
The scientists then returned the animals to the test chamber. At first, the rats scratched at their faces, indicating that they were interpreting the lights as touch. But after a month - as shown in these videos - the animals learned to associate the signal in their brains with the infrared source.
They began to search actively for the signal, eventually achieving perfect scores in tracking and identifying the correct location of the invisible light source.
One key finding was that enlisting the touch cortex to detect infrared light did not reduce its ability to process touch signals.