When neuroscientists used a monkey's thoughts to control computers, it was a huge breakthrough in mind-machine research. But harnessing brain waves has become even more complex now that humans are the subjects. Recently, researchers used the thoughts of one human's brain to control the physical actions of another. Really. As a panel of experts explained at the World Science Festival this week, brain-to-brain linkups are just getting started.

The field got its start in 1998 in the lab of Miguel Nicolelis, a Brazilian researcher working at Duke University. Before Nicolelis started experimenting with the brain, scientists were measuring the electrical output of a single neuron at a time. But Nicolelis and his colleagues began recording information from the brains of rats, where they discovered that to make their bodies move, rat brains would fire 48 neurons at a time. Believing that they could advance their understanding further, Nicolelis and his team then turned to monkeys.

They recorded 100 neurons firing at once in the brain of a monkey. Believing they might be able to take this data and use it to perform a task, the team connected a probe into the area of the monkey's brain that controlled for arm movement. Then they gave the monkey a game to play: Using a joystick, the monkey moved a dot around on a screen until it entered a circle in the center. When the monkey moved the dot into the correct location, she received a reward of juice. Once they recorded the brain patterns that resulted from the movement, the team took the joystick away. The monkey was now able to move the dot around simply by imagining it move.

"Somehow she figured out that she could just imagine. She realized this is the prototype of a free lunch," Nicolelis said. The innovation was the grandfather of the brain-to-brain interface. "This was the first time a primate's brain liberated itself from the body," he said.

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After Nicolelis's study, other neuroscientists began taking the work to humans. In 2013, Chantel Prat and Andrea Stocco, both researchers at the University of Washington Institute for Learning and Brain Sciences, wanted to see if they could send a message to control physical movement from one brain to another. Because it's a breach of research ethics to connect probes directly into a living human brain, they had to figure out how to do it using non-invasive techniques.

Using an electroencephalography (EEG) cap, which records brain activity, they positioned two researchers in separate areas of the campus. In one room a colleague, Rajesh Rao, played a videogame using his mind . Each time Rao saw an enemy he wanted to shoot in the game he would think about pressing a button. Across campus, Stocco sat with his back to the same video game while wearing noise-canceling headphones so he wouldn't know when to respond. On his head was a transcranial magnetic stimulation coil (a device that can emit a focused electrical current), which was positioned directly over the part of the brain that controlled the movement of his finger.When Rao thought about moving his finger, the signal was transmitted across campus to Stocco who, without any knowledge of it, would twitch his finger and trigger the game to shoot an enemy.

"The first time I didn't even realize my hand had moved. I was just waiting for something to happen," said Stocco.

That reaction, Prat says, is an important aspect of this science. "There is this idea that I would like to dispel. This is not the X-Men version of telepathy where you hear a disembodied voice. My brain would have no way of knowing that your thoughts are mine. Whatever shape [future brain-to-brain communication] takes is going to be very different than listening to someone's thoughts in your head."

"I don't think we will ever be able to broadcast from one brain to another the essence of the human condition."

The neuroscientists all agreed that, while this technology is still rudimentary there are implications for future uses. Nicolelis, for example, has adapted the brain-to-machine interface to help paralyzed patients walk by using their brain signals to control prosthetic devices. He says that over the two years he's been working with them several of his patients have recovered some sensory ability in their paralyzed lower limbs. "The conjunction of output to control device and feedback may have triggered axons that survived to start working again," he says.

Prat, who is especially interested in the differences between individual brains, believes that the technology could also eventually be used to improve learning by harnessing the EEG's ability to distinguishing between a brain that is focusing and one that is "zoning out." That way, perhaps in the future, when a "good learner" starts to focus on a learning task their brain can trigger someone who is not paying attention to focus in on the task at hand. Brain-to-brain communication, she says, may one day be especially good at transmitting a state of mind.

In the end the researchers agreed that despite the technology's many potential benefits one future we won't see is one in which you can connect your brain into a computer and download all the Earth's knowledge. According to Nicolelis, downloading massive amounts of data or mimicking telepathy will be impossible because the brain is just too complex.

"I don't think we will ever be able to broadcast from one brain to another the essence of the human condition. We don't even know how to record those things let alone broadcast them and then interpret that broadcast. We love analogies, metaphors, expecting things, and predicting things. These thing are not in algorithms. We're not going to be broadcasting my dreams to your head."

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