Two monkeys with tiny sensors in their brains learned to control a mechanical arm with just their thoughts, using it to reach for and grab...

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Two monkeys with tiny sensors in their brains learned to control a mechanical arm with just their thoughts, using it to reach for and grab food and to adjust for the size and stickiness of morsels when necessary, scientists reported Wednesday.

Scientists expect the technology eventually will allow people with spinal-cord injuries and other paralyzing conditions to gain more control over their lives.

The report, released online by the journal Nature, suggests that brain-controlled prosthetics, while not practical, are at least technically within reach.

In previous studies, researchers showed that humans who had been paralyzed for years could learn to control a cursor on a computer screen with their brain waves and that nonhuman primates could use their thoughts to move a mechanical arm or a small vehicle.

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The new experiment goes a step further. In it, the monkeys’ brains seem to have adopted the mechanical appendage as their own, refining its movement as it interacted with real objects in real time. The monkeys had their own arms gently restrained while they learned to use the added one.

“This study really pulls together all the pieces from earlier work and provides a clear demonstration of what’s possible,” said Dr. William Heetderks, director of the extramural-science program at the National Institute of Biomedical Imaging and Bioengineering. He was not involved in the study.

In the experiment by researchers from the University of Pittsburgh and Carnegie Mellon University, two macaques used a joystick to gain a feel for the arm, which had shoulder joints, an elbow and a grasping claw with two mechanical fingers.

Just beneath the monkeys’ skulls, the scientists implanted a grid about the size of a large freckle. It sat on the motor cortex, over a patch of cells known to signal arm and hand movements. The grid held 100 tiny electrodes, each connecting to a single neuron, its wires running out of the brain and to a computer.

The computer analyzed the collective firing of these 100 motor neurons, translating that into an electronic command and sending it to the arm, mounted flush with the left shoulder.

The scientists used the computer to help the monkeys move the arm, teaching them with biofeedback.

After several days, the monkeys needed no help. They sat in a chair, repeatedly manipulating the arm with their brain to reach out and grab grapes, marshmallows and other nuggets dangled in front of them. The snacks reached the mouths about two-thirds of the time.

On several occasions, a monkey kept its claw open on the way back, with the food stuck to one finger. At other times, a monkey moved the arm to lick the fingers clean or to push a bit of food into its mouth while ignoring a newly presented morsel.

The animals were apparently freelancing, discovering new uses for the arm, showing “displays of embodiment that would never be seen in a virtual environment,” the researchers wrote.

“In the real world, things don’t work as expected,” said the senior author of the paper, Dr. Andrew Schwartz, a professor of neurobiology at the University of Pittsburgh. “The marshmallow sticks to your hand or the food slips. … But the monkeys’ brains adjusted.”

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