Medical technology: A man paralysed from the neck down has shown he can open e-mail, control a TV and move objects with a robotic arm by thought alone.
The 25-year-old American patient, Matthew Nagle, had a computer-linked implant placed in his brain that enables him to operate devices just by thinking about it.
Brain-computer interfaces have been demonstrated before in humans and animals. But this is the biggest step taken so far towards developing "bionic" systems that can restore motor function in people who have lost control of their limbs.
In the 1970s television series The Six Million Dollar Man, scientists rebuilt the body of crash victim Steve Austin with bionic prosthetics controlled by his mind. At the time the concept was pure fantasy, but in the future thought-controlled replacement limbs could become real.
The results, described last week in the journal Nature, represent the culmination of decades of work. However, the scientists involved stress that the technology is still in its infancy.
Nagle, whose spinal cord was severed in 2001, received the implant at Rhode Island hospital in 2004. Known as the BrainGate Neural Interface System, it consists of an array of electrodes that record neural activity from the motor cortex of the brain.
Signals from the implant are decoded and processed by a computer, allowing them to be translated into movement commands.
First, Nagle learned to move a computer cursor by focusing his thoughts on the task. Later, during 57 trial sessions at the New England Sinai hospital and rehabilitation centre in Massachusetts, he expanded his repertoire of thought control.
He was able to open e-mails, draw circular shapes on a computer screen, play a simple video game, and change the channel and adjust the volume on a television. Eventually, he could also open and close the fingers of an artificial hand and use a robotic arm to grasp and move objects.
A second patient, aged 55, who had a sensor implanted at the University of Chicago in April 2005, was able to move a computer cursor for three months, until his implant malfunctioned.
Prof John Donoghue, who led the research and heads the brain science programme at Brown University, says: "The results hold promise to one day be able to activate limb muscles with these brain signals, effectively restoring brain-to-muscle control via a physical nervous system."
Prof Donoghue is chief scientific officer at Cyberkinetics Neurotechnology Systems, which developed the implant.
Previous attempts at linking brains to computers have had limited success, such as getting patients to move a cursor to the left and right. Experiments have also been conducted with less invasive techniques using sensors attached to the scalp, but these take months of training to use.
Nagle adapted to the BrainGate system in minutes, and was able to talk while using it. The implant consists of a pill-sized sensor measuring 4mm across and containing 100 tiny electrodes, each thinner than a human hair.
It is placed on the surface of the motor cortex, the area of the brain responsible for voluntary movement. The electrodes penetrate about 1mm into the brain, where they pick up electrical signals from nearby neurons. These are transmitted through thin gold wires to a titanium pedestal protruding about an inch above the patient's scalp. A cable connects the pedestal to the computer. In the future it is hoped that wireless systems will avoid the problem of having to use bulky cables.
The patient learns how to operate devices simply by imagining a task being carried out. Dr Richard Penn, the University of Chicago neurosurgeon who implanted the sensor in the second patient, says: "This is the strangest, most interesting surgery I've ever done. Not the technical stuff, but the data that we get from the neurons firing in different patterns when you're thinking in different ways. And seeing it is only the beginning."
Experts had not been certain that the brain's limb-control signals could still be found years after a paralysing spinal injury. But such doubts were put to rest by the success with Nagle, who was injured three years before his surgery. "We're finding that, even years after spinal cord injury, the same signals that originally controlled a limb . . . can be utilised," says Dr Leigh Hochberg, a neurologist from Massachusetts general hospital who was a member of the research team.
Technical obstacles remain, such as the variation in individual responses to the implants and a tendency for the sensors to become less efficient over time. To restore limb function in any meaningful way, scientists must also work out how the body tells the brain where its limbs are positioned in space. This is done through a little-understood sense called "proprioception".
However, enormous challenges have been overcome - in particular being able to "listen" to large groups of brain cells firing together between 20and 200 times a second.
Dr Francisco Sepulveda, co-ordinator of the brain-computer interfaces research group at the University of Essex, says: "The paper in Nature is important in that it shows the first commercial attempt to use... brain signals to control devices just by thinking. This technology will have immense impact for disabled individuals in the future."