Two rats thousands of miles apart have successfully communicated through thought alone to solve a series of problems.
The team says the results show it could in future be possible to link multiple brains to form what they call an organic computer, allowing the sharing of motor and sensory information among groups of animals.
"Our previous studies with brain-machine interfaces had convinced us that the rat brain was much more plastic than we had previously thought," says Miguel Nicolelis of Duke University School of Medicine.
"In those experiments, the rat brain was able to adapt easily to accept input from devices outside the body and even learn how to process invisible infrared light generated by an artificial sensor. So, the question we asked was, 'if the brain could assimilate signals from artificial sensors, could it also assimilate information input from sensors from a different body?'"
To find out, the researchers first trained pairs of rats to solve a simple problem: to press the correct lever when an indicator light above it switched on, delivering a sip of water. They next connected the two animals' brains via arrays of microelectrodes inserted into the area of the cortex that processes motor information.
One of the two rodents - designated the 'encoder' - received a visual cue that showed it which lever would deliver the reward. Once it pressed the right lever, a sample of the brain activity that coded its behavioral decision was translated into a pattern of electrical stimulation that was delivered directly into the brain of the second rat, known as the 'decoder'.
The decoder rat had the same types of levers in its chamber, but didn't receive any visual cue to indicate which lever to press. To solve the problem, it would have to rely on the cue transmitted from the encoder via the brain-to-brain interface.
And solve the problem it did. The decoder rat ultimately achieved a maximum success rate of about 70 percent - not far off the maximum success rate of 78 percent, limited by success rates of sending signals directly to the decoder rat's brain.
Importantly, the communication was two-way. For instance, the encoder rat didn't receive a full reward if the decoder made a wrong choice.
"We saw that when the decoder rat committed an error, the encoder basically changed both its brain function and behavior to make it easier for its partner to get it right," says Nicolelis.
"The encoder improved the signal-to-noise ratio of its brain activity that represented the decision, so the signal became cleaner and easier to detect. And it made a quicker, cleaner decision to choose the correct lever to press. Invariably, when the encoder made those adaptations, the decoder got the right decision more often, so they both got a better reward."
Similar results were found when pairs of rats were trained to distinguish between a narrow or wide opening using their whiskers.
To emphasize their success, the researchers placed an encoder rat in Brazil and transmitted its brain signals over the internet to a decoder rat in Durham, NC - and found that the two rats could still work together.
"So, even though the animals were on different continents, with the resulting noisy transmission and signal delays, they could still communicate," says post-doctoral fellow Miguel Pais-Vieira. "This tells us that it could be possible to create a workable, network of animal brains distributed in many different locations."
Researchers are now working on experiments to link multiple animals cooperatively to solve more complex behavioral tasks.
"We cannot predict what kinds of emergent properties would appear when animals begin interacting as part of a brain-net. In theory, you could imagine that a combination of brains could provide solutions that individual brains cannot achieve by themselves. Such a connection might even mean that one animal would incorporate another's sense of self," says Nicolelis.
"In fact, our studies of the sensory cortex of the decoder rats in these experiments showed that the decoder's brain began to represent in its tactile cortex not only its own whiskers, but the encoder rat's whiskers, too."