For scientists building devices that would allow the paralyzed to interact with their environment (as well as those wishing to understand the workings of the brain), it’s important to understand that using two virtual arms is a more complex proposition than using each arm separately and multiplying by two. But with a little help and a lot of technology, a new study shows, even a monkey can be taught to do it.

The latest account of a brain-machine interface that could give a quadriplegic two good hands was published Wednesday in the journal Science Translational Medicine. The article describes a process by which two monkeys were taught to move two arms on a computer terminal in front of them, using only their thoughts.

One of the monkeys achieved that feat by first manipulating two virtual arms, projected on a screen in front of him--with his own hands on a pair of joysticks. But the other did so without ever actually using his hands. Instead, the second monkey watched a projected image in which two arms move around a field and manipulate virtual “buttons.” That vicarious experience alone produced a complex pattern of neuronal firing in the monkey’s brain. Later, when the monkey reenacted that distinctive firing pattern by thinking about how he would move his arms, the two virtual arms carried out the command.

That accomplishment holds out the prospect that even patients with no control over their limbs might one day be able to choose a virtual arm--or two--and send them forth to turn on lights, punch in a phone number or type a letter.

In the latest study, researchers found that it would take neural “recordings” from many different clusters of neurons across the brain to replicate the act of moving both arms. In all, researchers listened in on and recorded the activity of about 500 neurons distributed across the frontal-parietal lobes of each animal to create the neural signature of each distinct movement of the arms.

That reproducing the movement of both arms would involve such a widely distributed ensemble of neurons was a surprise even to the researchers involved, said lead author Dr. Miguel Nicolelis of Duke University.

Those neural signatures became the “commands” that directed the virtual arms to move, even when the monkeys’ arms were gently immobilized at their sides. When a computer program detected the telltale signature of a monkey wishing to use his left arm to reach out and press on a red square on the screen, it executed that movement virtually.

The research, carried out largely at Duke University’s Center for Neuroengineering, is part of a broad field of research into “neural prosthetics” and “brain-machine interfaces” that could restore function lost to illness or injury. Some hope that the same technologies could someday extend normal human capabilities as well.

The work is led by Nicolelis, a Brazilian-born physician who heads an international consortium of brain and computer scientists, engineers and prosthetics experts intent on building a full-body prosthesis for the paralyzed. The “Walk Again Project” aims to allow a person with paralysis to use technology and his or her thought patterns to bypass a severed spinal cord. To that end, Nicolelis and his team have pioneered a wide range of brain-machine interfaces, tested first on nonhuman primates, that could restore movement, control and touch to those paralyzed below the neck.

The consortium is on course to unveil its exoskeleton in June in Brazil, at the opening of the Soccer World Cup, and is currently training a pool of patients in Brazil in the brain-machine interfaces that are at the heart of the project. The culmination of a project Nicolelis calls “the Brazilian moonshot,” one of those paralyzed patients will be chosen to walk onto the field and open the Games with a decisive kick of a ball.

The two-armed monkey study is “one more step” toward achieving that goal, Nicolelis said in an interview Wednesday.

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