The rat has an IR sensor affixed to her head, with the pink cone coming from the detector indicating the “receptive field” of the detector (that is, the locations in space within which an IR source would activate her detector). It shows two IR sources in her environment associated with different cues (cheese and water). The top half of the picture shows the flow of information from the IR detector, through the computer for processing, and out to the microstimulator which generates microcurrrents in the rat’s brain. (Credit: Laboratory of Miguel Nicolelis, Duke University)
In 2004, Eric Thomson, fresh from finishing his Ph.D. in neuroscience at the University of California, San Diego, was seeking his next challenge.
He visited a variety of laboratories across the country, but wasn’t quite enthused with the potential research avenues available for exploration.
His interests lay in touch processing, specifically with freely moving laboratory rats. However, many of the labs he visited were resistant to that kind of work, opting to run experiments on anaesthetized and fixed animals rather than freely moving creatures. Then, he found Dr. Miguel Nicolelis, principal investigator of Duke University School of Medicine’s Nicolelis Lab.
“He was back then doing pretty cutting-edge work, and implanting multi-electrode arrays in freely moving rodents, doing interesting behavioral tasks,” said Thomson.
In Nicolelis, Thomson found someone who wasn’t afraid to ask tough questions and tackle complex issues. He signed up for the ride.
Twelve years later, Thomson is still there. He’s now a research scientist with the Nicolelis Lab, which is still on the cutting edge and pushing boundaries.
A “scientific playground,” as described by Thomson, the Nicolelis Lab is a fast-paced environment led by someone who is often a few steps ahead of his researchers.
“We try to keep up,” Thomson said. “I’ve never really worked with anyone quite like him. That’s the reason I decided to stay with the lab.”
Located on Duke University’s campus, the Nicolelis Lab is home to an array of experiments. One of Thomson’s colleagues is working on a spinal stimulation technique for Parkinson’s disease, another is experimenting with brain-to-brain interfaces for rats, and others are enabling primates to control exoskeletons and wheelchairs with their neural activity.
Meanwhile Thomson, who relishes the nitty-gritty of an experiment, is endowing rats with the ability to sense infrared light.
A handful of years ago, Nicolelis approached Thomson with an idea. He wanted to see if it was possible to bypass a rat’s normal sensory transducers, which transfer information from the five senses to the brain. Using off-the-shelf components, Thomson designed the experiment. He attached a single infrared detector to the heads of adult rats. After sensing infrared light, the detectors would send the information to the rats’ somatosensory cortex, a receptive area for touch.
Rats have a particularly advanced somatosensory cortex, Thomson said.
“Just like we have a disproportionally large part of our cerebral cortex devoted to visual processing, they have a disproportionally large part of their cortex devoted to whisker processing,” he said. “So, it’s a real nice model system for somatosensory processing in general.”
In the experiment, the rats were given a myriad of reward ports to choose from, one of which would display infrared light.
Days went by, and then weeks. The rats didn’t seem to understand there was an association between the correct port and the microstimulation they felt in their brains. Error tones buzzed, distressing both the rats and Thomson.
“We almost gave up on this project,” Thomson said. “We had to tweak so many different things.”
Then, a breakthrough. After about 30 days, the rats started sweeping the sensors mounted to their heads, as if searching the environment with “a cyclopean eye,” Thomson said.
“They were finally trying to forage and sample the (infrared) environment, rather than randomly going to those ports like they didn’t know what was going on,” he added. “Kind of like a heat seeking missiles, they honed in on their target.”
Nature Communications published a paper on the experiment in 2013. For Thomson, the success proved that the adult rat’s brain was plastic enough to reroute new sources of information to the somatosensory cortex.
In the succeeding years, Thomson and colleagues decided to increase the experiment’s capacity. This time around, they implanted four infrared detectors in the rats’ brains. Thomson was unsure whether the higher amount of microstimulation would help them learn more quickly, or just overwhelm and confuse them. “This wasn’t just an idle worry,” he said. “This was actually a very real fear for me.”
But the rats rose to the occasion. Instead of 30 days, they learned how to perform the task in around 4.5 days.
“I was frankly astounded because I’ve never had my rats learn any task in four and half days, much less learn to discriminate a completely new sensory modality that they’ve never experienced before,” Thomson said. It suggests that the brain “is very happy to exploit and take advantage of that increase in information.”
While it appeared like the rats were using the infrared detector like a new visual modality, Thomson was unsure whether that was actually the case. “Are they experiencing it like vision?” he wondered. Or “are they experiencing it like touch and they’re just associating a tactile sensation with this stuff in the environment? I have no idea.”
The issue raises philosophical questions regarding subjective experience. And since one can’t ask a rat what they’re experiencing, one can only speculate. It’s an even bigger unknown for humans.
According to Grazyna Palczewska, the director of medical device development for biotech company Polgenix Inc., the human visible spectrum is limited to wavelengths between 400 and 720 nanometers. But previous studies have indicated humans may be able to sense infrared wavelengths. In 1947, researchers reported that at wavelengths above 800 nm, rod photoreceptors became more sensitive than cones and allowed people to see infrared as white light.
In 2014, Palczewska and her colleagues published a study in Proceedings of the National Academy of Sciences regarding their own research into human infrared vision. ”We designed an experiment where two sources of light were intermittently shot into the human eye,” Palczewska said.
The various stimulus wavelengths, shot by lasers at infrared wavelengths, were perceived by study participants at visible spectrum wavelengths. According to the Washington University in St. Louis, whose faculty contributed to the study, “packing a lot of photons in a short pulse of the rapidly pulsing laser light makes it possible for two photons to be absorbed at one time by a single photopigment, and the combined energy of the two light particles is enough to activate the pigment and allow the eye to see what normally is invisible.”
Though under certain circumstances humans can glimpse infrared wavelengths, it’s nothing like the science fiction Predator-style vision.
For Thomson, the work he’s doing has much bigger implications for prosthetics. He believes the research might pave the way for prosthetic limbs that can transmit sensations of touch and pressure to the user.
“That’s very difficult to do without some kind of sensory feedback,” he concluded. “We want to give users a sense of tactile feedback with prosthetic limbs, so they can have better facility with the limb and perhaps a better sense of ownership of the limb.”
