May 21 2012

Mental Control of a Robotic Arm

Another step has been taken in the research to develop practical brain-machine interfaces (BMI). The basic concept, as I have discussed previously, is to read electrical activity from the brain, then use a computer to interpret that electrical activity and use it to control something, which can be a cursor on a computer screen, a robotic arm, or any other electrical device.

In order for this technology to be useful it has to be possible for the person whose mental activity is being monitored to learn how to control the cursor or robot with their thoughts. Previous research shows that brain plasticity allows for such BMI prosthetics to feel natural – in other words, it is possible to learn how to control external devices through a BMI just as if they were a part of the body.

There are two basic ways to read electrical activity, through implanted sensors or through scalp sensors. The implanted sensors are much better because they are in direct contact with the brain, but then there needs to be some way for the sensor to communicate outside the brain. This is done currently with wires. This, of course, is an invasive procedure and can have complications. The scalp sensors are much safer and easier to apply, but the resolution is much lower as the electrical activity of the brain is attenuated by the skull and scalp – so it’s like looking through a thick pane of foggy glass. For this reason it seems that the future of BMI will be implantable sensors.

A new paper published in Nature reports on two patients who are paralyzed due to brain injury who have had sensors implanted in their brain. One patient, who is a paralyzed in all four limbs due to stroke, had a 96 channel sensor implanted in her motor cortex five years ago. This demonstrates the long term viability and safety of such implants. Further, she has had quadraparesis for 15 year. This means there was ten years between her injury and the implanting of the sensors. This is not a small point – it means that the motor cortex remains viable years after it has been disconnected from the muscles it controls. It is important to note that the subjects did not have injury to their motor cortex itself, but to structures farther downstream – either the brainstem or spinal cord.

One subject, shown in the video, was able to control a robotic arm with enough control to reach out and pick up a cup, bring it to her mouth, and then drink through a straw. The level of control is quite impressive, although still much slower and more clumsy than normal limb movement. The article notes that she learned to do this without explicit training.

I have been fascinated with this line of research for years and as each new study comes out the future of BMI looks brighter and brighter. Every critical step in the process has been demonstrated: sensors can function for years in the brain, they can read brain activity, relevant brain areas remain active even years after injury, the brain’s plasticity can adapt to the sensors which can even feel “natural”, and subjects implanted with such devices can learn to control them well enough to perform specific functional tasks, like eating, drinking, or communicating via a computer. I can’t think of any significant component to the BMI system that has not already been demonstrated in humans.  The BMI paradigm works.

Researchers will still do basic research on BMI to gain more knowledge and familiarity with the details of how such systems can work, but essentially at this point the technology just needs to mature. One obvious improvement will be larger sensors that are able to read a larger portion of brain activity with more sensors. Computer hardware to interpret these signals does not seem to be a limiting factor in any way, but of course continued computer advances can only help. Software development also seems to be advanced enough to read and interpret the signals, but this too will likely continue to incrementally advance. Once you can control a computer, the whole world of information and communication is available. And robotics itself continues to incrementally advance, but is already at the point that useful robotic arms are available.

In addition to controlling a computer screen and controlling a robotic arm, a third possible application of BMI technology is to control the person’s own limbs. This has already been accomplished also – another team used a BMI on monkeys whose spinal cords were temporarily paralyzed. The monkeys were able to control their own arm and hand through a BMI that read their motor cortex activity and then bypassed the spinal cord to directly stimulate the muscles of the arm. The monkeys were able to perform tasks like picking up  a ball. Similar research has yet to be done in humans, but it should work just as well.

It may seem like this would be the ultimate goal of BMI technology – to restore control of one’s own body by bypassing an injury – there is a significant limitation to this approach. Muscles that have lost their nerve supply also lose their function. With an upper motor neuron lesion (brain or spinal cord above the nerves cell to the relevant muscles) muscles will become spastic with involuntary contractions. For lower motor neuron lesions (spinal cord region that supplies the muscles or the peripheral nerves that connect to the muscles) the muscle will become severely atrophied. Supplying electrical stimulation to the muscles does not reverse or prevent these effects, especially the atrophy. Muscles need specific trophic factors from the nerve endings to stay alive and healthy, electrical stimulation is not enough. There would be no point, therefore, in controlling either a spastic or extremely atrophied limb.

These are non-trivial issues that have been researched for years, without a good solution. Controlling a robotic limb, therefore, seems like a more viable option, at least for the foreseeable future. The robot limbs, however, could be designed like an exoskeleton to wear over a subject’s own limbs. This could theoretically enable them to walk, and also could keep their limbs active (a form of physical therapy) to help prevent contractures of the joints.

We are already at the point where BMI technology can be useful to a paralyzed individual. For those who are essentially “locked in” the ability to communicate or manipulate their environment at all is extremely useful. I would like to see this technology go beyond the research phase and be developed for use in  such patients. Meanwhile it will be interesting to see this technology continue to develop.

Like this post? Share it!

24 responses so far