By Julie Steenhuysen Wed May 28, 3:20 PM ET

CHICAGO (Reuters) - Using only its brainpower, a monkey can direct a robotic arm to pluck a marshmallow from a skewer and stuff it into its mouth, researchers said on Wednesday.

"They are using a motorized prosthetic arm to reach out, grab and bring the food back to their face," said Andrew Schwartz of the University of Pittsburgh School of Medicine, whose study will appear in an upcoming issue of the journal Nature.

Schwartz said the technology behind this feat may lead to brain-powered prosthetic limbs for people with spinal cord injuries or disabling diseases that make such simple tasks impossible.

Until now, such brain-machine interfaces have been used to control cursor movements on a computer screen. Schwartz and colleagues wanted to apply the technology to real-world tasks.

The monkey guides the robot arm the same way it does its natural limbs, through brain signals.

Schwartz' team picks up those signals through an array of microelectrodes half the size of a thumbtack that has been implanted in the monkey's brain. These signals are amplified and relayed to a computer that operates the robotic arm.

Schwartz said his team has learned that certain motor neurons fire rapidly when the monkey wants to move a certain way. "What is important is each neuron seems to have a preferred direction," Schwartz said in a telephone interview.

"One cell will fire a lot if you move upward. Another cell will fire a lot if you move to the right. All you really need to do is listen to these neurons at the same time to determine which direction the animal wants to move in," he said.

COMPUTER CONTROL

"We record those patterns of action potential, interpret them with a computer and extract the monkey's intention to move. That serves as a control signal to the robot."

Schwartz said it takes about three days for a monkey to learn to operate the arm, and they continuously improve.

So far, they have trained two monkeys to feed themselves with the robotic arm. The monkeys sit in a chair with their arms gently restrained in sleeves that keep them from simply grabbing the food on their own. "These animals will just relax their arms as they control these devices," Schwartz said.

The monkeys appear to enjoy the task. "They sure like eating their marshmallows." Sometimes the team will use pieces of apple, orange or zucchini. "Just about anything we can that doesn't make too big of a mess," Schwartz said.

The ultimate goal is to develop a brain-powered prosthesis that can restore near-natural function to an amputee or person with a spinal cord injury.

But first, they want to refine the system. The next step is to develop an operating wrist and jointed fingers to add dexterity to the device.

"If you look at what these patients really need, they need to be able to use their fingers to increase their quality of life. They need to button shirts and pull zippers and things like that," Schwartz said.

The researchers must overcome several engineering challenges, including developing more durable electrodes that do not lose their signal over time, but Schwartz believes such devices are feasible.

"We're learning more and more about brain function as we do this," he said.

(Editing by Maggie Fox and Eric Beech)