Creating a "superbrain" of connected minds, scientists on Thursday said they had enabled a rat to help a fellow rodent while the animals were a continent apart but connected through brain electrodes.
With electrodes imbedded in its cortex, a rat in a research institute in Natal, Brazil sent signals via the Internet to a counterpart at a university lab in Durham, North Carolina, helping the second animal to get a reward.
The exploit opens up the prospect of linking brains among animals to create an "organic computer," said Brazilian neurobiologist Miguel Nicolelis.
It also helps the quest to empower patients stricken with paralysis or locked-in syndrome, he said.
"We established a functional linkage between two brains. We created a superbrain that comprises two brains," Nicolelis said in a phone interview with AFP.
Published in the journal Scientific Reports, Nicolelis' team gave basic training to thirsty rats, who had to recognise lights and operate a lever to get a reward of water.
They then implanted ultra-fine electrodes in the rats' brains, which were linked by a slender overhead cable to a computer.
In a glass tank in Natal, the first rat was the "encoder," its brain sending out a stream of electrical pulses as it figured out the tricks for getting the reward.
The pulses were sent in real time into the cortex of the second rat, or "decoder," rat, which was facing identical apparatus in a tank in North Carolina.
With these prompts from its chum, the decoder rat swiftly found the reward in turn.
"The pair of animals collaborated to solve a task together," said Nicolelis.
What the second rat received were not thoughts, nor were they images, Nicolelis said.
When the encoder rat achieved various tasks, the peaks in his brain signals were transcribed into a telltale pattern of electronic signals that were received by the decoder rat.
Once the rat recognised the usefulness of these patterns, they became incorporated into its visual and tactile processing.
"The second rat learns to recognise a pattern, a statistical pattern, that describes a decision taken by the first rat. He's creating an association of that pattern with a decision," said Nicolelis.
"He may be feeling a little tactile stimulus, but it's something that we don't know how to describe because we cannot question the subject."
The linkage "suggests we could create a brain net, formed of joined-up brains, all interacting," the scientist said, hastening to stress that such experiments would only be conducted on lab animals, not humans.
"If you connect several animal brains, rat brains or primate brains, you probably could be creating an organic computer that is a non-Turing machine, a machine that doesn't work according to the Turing design of all the digital computers that we know. It would be heuristic, it wouldn't use an algorithm, and it would uses probabilistic decision-making based on organic hardware."
Still unclear is how the decoder animal incorporates the encoder's signals into its mental space, a phenomenon called cortical plasticity.
"We basically show that the decoder animal can incorporate another body as an extension of the map that the animal has in it's own brain," said Nicolelis, adding, though: "We don't know how this is done."
Nicolelis carries out research at Duke University in Durham and at the Edmond and Lily Safra International Institute for Neuroscience of Natal, or ELS-IINN.
A decade ago, he leapt to prominence for pioneering work in having lab monkeys move a robotic arm through brain impulses.
The latest work should help this, he said: "We are learning ways to interact with and send messages to the mammalian brain that will be fundamental for our goals of medical rehabilitation."
His next goal is to have a paraplegic patient give the official kickoff to the 2014 World Cup in Brazil, using a brain-machine interface to activate an artificial limb.
Brain signals about completing a simple task were transmitted from one rat to another
Scientists have connected the brains of lab rats, allowing one to communicate directly to another via cables.
The wired brain implants allowed sensory and motor signals to be sent from one rat to another, creating the first ever brain-to-brain interface.
The scientists then tested whether the rat receiving the signal could correctly interpret the information.
As the ultimate test of their system, the team even linked the brains of rats that were thousands of miles apart.
Details of the work are outlined in the journal Scientific Reports.
Professor Miguel Nicolelis and his team at Duke University Medical Center in North Carolina built on their previous work with brain-machine interfaces.
In a study published earlier this month, the researchers implanted electrodes in the part of the rat's brain that processes tactile information and attached these to infrared sensors - effectively allowing the rat to "touch" infrared light.
In their latest study, the scientists wanted to test whether the systems they had developed could be used to establish a new artificial communication channel between animals.
"Until recently we used to record this brain activity and send it to a computer... and the [computer] tells us what the animal is going to do," Prof Nicolelis told the BBC's Science in Action programme.
"So we reasoned, if we can do that with a computer, could another brain do that?"
The researchers first trained pairs of rats to solve a simple problem - pressing the correct lever when an indicator light above the lever switched on, to obtain a water sip.
The researchers then placed the rodents in separate chambers and connected their brains using arrays of microelectrodes - each roughly one hundredth the diameter of a human hair - inserted into the area of the cortex that processes motor information.
One rat is designated the "encoder", who will receive a visual clue, the other is the "decoder", who will not
One rat was designated as the "encoder". Once this rat pressed the correct lever, its brain activity was delivered as electrical stimulation into the brain of the second rat - designated the "decoder".
The decoder rat had the same types of levers in its chamber, but it did not receive any visual cue indicating which lever it should press to obtain a reward.
In order to receive the reward, the decoder rat would have to rely on the cue transmitted from the encoder via the brain-to-brain interface.
The team members then conducted trials to determine how well the decoder animal could decipher the brain input from the encoder rat to choose the correct lever. The decoder rat ultimately achieved a maximum success rate of about 70%.
Although the information was transmitted in real time, the learning process was not instantaneous.
"[It] takes about 45 days of training an hour a day," said Prof Nicolelis.
"There is a moment in time when... it clicks. Suddenly the [decoder] animal realises: 'Oops! The solution is in my head. It's coming to me' and he gets it right."
There was also a feedback system, denying the encoder rat an extra reward if the decoder rat did not press the correct lever.
The encoder rat's brain signals then became clearer, giving the decoder a greater chance of interpreting the message correctly, Prof Nicolelis noted.
He explained: "Basically [the second rat] is working as... a biological computer."
One replication of the experiment successfully linked a rat at Duke with one at the University of Natal in Brazil. Nicolelis foresees eventually extending the system to larger numbers of animals. "We are already building the setup... You could actually have millions of brains tackling the same problem and sharing a solution."
And he also thinks the idea could be extended to humans.
"We will have a way to exchange information across millions of people without using keyboards or voice recognition devices or the type of interfaces that we normally use today," he said.
"I truly believe that in a few decades… we will know what it is to communicate in that way."
But Prof Nicolelis is clear that this depends on the development of non-invasive techniques to share information between human brains.
Prof Christopher James, an expert in neural engineering at the University of Warwick, who uses non-invasive techniques in his own research, explains that it is currently not possible to put information into a brain using just the surface of the scalp.
"If you want to get information into the brain, then putting electrodes right at the brain sites is the way to do it. However, it's clearly very invasive," said Prof James.
He added that the invasive nature of the research raised ethical questions: "It's very, very interesting isn't it? Because in humans you'd obviously get informed consent in doing this."
Prof James explained: "It's an exciting paper which basically shows that it is possible to take information out of the brain, and it is possible to take information and pump it into the brain.
"What this shows is that the technology is here. And the sort of things we should be talking about is: Why are we doing this, and what do we hope to get out of it?"