20 October 2008

Neuroprosthesis allowed the monkey to control the paralyzed arm

Eberhard Fetz and his colleagues from the University of Washington built a system with which a monkey learned to control the muscles of his arm, which had no direct connection with the brain. The experience opens up tempting prospects for paralyzed patients with spinal column injuries.

Experimental macaques (Macaca nemestrina) were taught a very simple computer game, which was controlled via a joystick. Winning was accompanied by a reward. At the same time, electrodes were implanted into the monkeys' brains, which removed activity signals from neurons of the motor cortex. It was these signals that the computer converted into the movement of the cursor on the screen (the monkey believed that the joystick was connected to the computer, and industriously moved the brush).

Then the scientists equipped the monkeys' wrist with an electrical stimulator and connected it to a computer that received a signal from the motor cortex. The normal path of nerve signals from the brain to the arm was temporarily blocked (by injection of a special drug). And finally, we switched the video game now really to the joystick.

The monkeys were not at all discouraged by all these changes — they continued to play as usual, proving that their brain successfully controls the paralyzed limb through a computer interface.

Previously, some scientific groups showed the possibility of removing signals from the motor cortex of the brain, which then controlled the robot manipulator. Others demonstrated the possibility of functional electrical stimulation of certain muscles, controlled by a computer (without the participation of the brain). But for the first time, as reported PhysOrg.com , both of these approaches were combined.

Technology Review provides stunning details of the experiment. The fact is that the researchers did not know in advance exactly which neurons of the motor cortex they needed, and installed the electrodes at random (in several experiments in different ways).

It turned out that all the neurons in the motor cortex that the researchers tested could be successfully used to control this video game.

Even those nerve cells that initially had nothing to do with the movement of the wrist, after a short training, were successfully adapted to control the hand. At the same time, the monkey could provide independent control over the flexor and extensor muscles.

As a rule, hand movements — even the contraction of one muscle — are the result of the work of not one neuron in the cortex, but the coordinated actions of many neurons. Previously, researchers have succeeded a lot in capturing such complex signals, first decoding them, and only then converting them into the same cursor movements on the screen (this was taught to both monkeys and humans).

But Fetz took a different path: "We only created connections between individual neurons in the brain and individual muscles. And let the monkey find out for himself how to use this connection," the scientist explained.

Experience shows that neurons in the motor cortex do not have clearly predetermined functions, and that the cortex has great flexibility in adapting certain neurons to control limbs.

This means that future medical implants for paralyzed people may be much smaller and simpler than expected. After all, they will not have to decipher complex patterns of activity of the community of neurons (which requires decent computing power).

By connecting the cells of the motor cortex with the muscles at random, the doctors will simply have to wait until the patient learns to properly control his limbs. Judging by the monkeys, such a device can be relatively fast.

However, it will take another ten years from the current experiments on macaques to the appearance of this technology in medical practice, predicts Professor Fetz.

Details of the experience can be found in the article by the authors of the experience in Nature.

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