Our world is determined by the limits of our five senses. We can't hear pitches that are too high or low, nor can we see ultraviolet or infrared light—even though these phenomena are not fundamentally different from the sounds and sights that our ears and eyes can detect. But what if it were possible to widen our sensory boundaries beyond the physical limitations of our anatomy? In a study published recently in Nature Communications, scientists used brain implants to teach rats to “see” infrared light, which they usually find invisible. The implications are tremendous: if the brain is so flexible it can learn to process novel sensory signals, people could one day feel touch through prosthetic limbs, see heat via infrared light or even develop a sixth sense for magnetic north.
Miguel Nicolelis, a neurobiologist at Duke University, and his colleagues trained six rats to poke their nose inside a port when the LED light above it lit up. Then the researchers surgically attached infrared cameras to the rats' head and wired the cameras to electrodes they implanted into the rats' primary somatosensory cortex, a brain region responsible for sensory processing. When the camera detected infrared light, it stimulated the animals' whisker neurons. The stimulation became stronger the closer the rats got to the infrared light or the more they turned their head toward it, just as brain activation responds to light seen by the eyes. Then the scientists let the animals loose in their chambers, this time using infrared light instead of LEDs to signal the ports the rats should visit.
At first, none of the rats used the infrared signals. But after about 26 days of practice, all six had learned how to use the once invisible light to find the right ports. Even after months of doing so, the rodents were able to respond to whisker neuron stimulation in addition to the infrared light, which suggests that sensory neurons can, when necessary, respond to multiple types of cues. This approach could help scientists create “sensory channels” for prosthetics users that provide constant sensory feedback to and from artificial limbs, facilitating control. The findings also suggest that the human brain can handle an expanded sensory repertoire—that we might one day be able to see, hear, touch and smell what we now cannot.