Bionic brain chips kindle hope for the paralysed (2)
By Sam Ejike Okoye
COULD the electronics-brain partnership approach work in humans? There seem to be no fundamental obstacles, and Donoghue plans to test the proposition in the new BrainGate trials, using his chip to control a limb through FES. If successful, it will represent a milestone in the development of such treatments.
Direct electrical stimulation of muscles using FES is unlikely to be the final solution, however. This direct approach uses a relatively powerful electric current applied to large areas of tissue, producing fairly clumsy movements. A more elegant method, some claim, is to send the impulse along the existing healthy nerves. That would require smaller local currents, delivered with greater precision, to finer regions of the muscle tissue, which should allow more subtle control.
Indeed, as a bonus, nerve stimulation could simplify some of the demands placed on a brain chip. That is because for many rhythmic activities, like breathing, walking and crawling, the brain simply sends a command signal and it is the spinal cord's in-built systems that orchestrate the fine movements of each muscle. So, if the healthy sections of a damaged spinal cord have retained their ability to control movement, the electronic chip could transmit the brain signal around the broken connection but leave the muscular orchestration to the spinal cord.
In this case, a brain chip would simply beam the message to a second device implanted in the spine below the break, which would then stimulate the spinal cord. The chips could simply transmit the information around the break, leaving the undamaged sections of the spinal cord to orchestrate the muscles. That could Òdramatically simplify the control signals needed from the brain,Ó says Duke University in Durham, North Carolina, team member Chet Moritz, since for these repetitive tasks the brain chip would just decode and transmit an umbrella command. Such simplification should make the chips less likely to fail - an important consideration when the only way to replace the chips is through invasive surgery - and also reduce their power consumption.
Using this principle in 2002, Vivian Mushahwar, now at the University of Alberta in Edmonton, Canada, plugged four electrodes into a catÕs spinal cord and delivered signals that mimicked the brain's command to walk. Sure enough, the cat made stepping motions. However, simply relaying the messages across a break in this way would not help the worst injuries in which the spinal cord has lost its ability to coordinate muscles. In these cases, to minimize the size of the brain chip and the burden placed on it, the muscular orchestration would need to come from either the chip implanted in the spinal cord, or an external device that communicates wirelessly with the chips in the brain and the spine.
Calculating exactly which nerves to stimulate and in what pattern is no easy task, but the first demonstration of an artificial Òcentral pattern generator (CPG)Ó was reported last year, when Mushahwar and colleagues at Johns Hopkins University in Baltimore, Maryland, successfully tested such a chip on a cat. With coordination coming solely from an external CPG chip connected to a handful of electrodes that stimulated the cat's spine, the animal was able to walk. In this experiment, the team was simply testing the CPGÕs ability to orchestrate movement as an alternative to FES, so the trigger came from a manual switch and not the cat's brain. The next hurdle will be to use the CPG in conjunction with a neural chip.
While this CPG chip only dealt with the action of walking, in humans an additional external chip might also offload some of the processing from the brain chip for non-repetitive motions like clenching a fist or raising a hand. The brain doesn't necessarily produce an umbrella command for all of these movements, so the neural implant would still need to detect a more complicated signal, but the external chip could at least perform some of the processing to decode and relay these commands to the relevant electrodes.
For many patients, technology like this would only solve half the problem, however. Paralysed people who have lost feeling as well as movement in their limbs would need two-way systems to pass sensations back to their blain. This information could come from artificial sensors, but ideally the chip would read sensations from existing nerves and relay them to chips that stimulate the areas of the brain that process tactile information.
Although work has been slower in this area, there is good evidence it will one day be possible. Carmena, for instance, who is now at the University of California, Berkeley, recently stimulated a ratÕs brain to, feel sensations from some Òvirtual whiskers,Ó causing it to move as if its own whisker's had really brushed against an object. Similar technology could one-day relay, tactile information to human brains.
If these advances in brain-chip, capability are to be exploited, the researchers still need to ensure that the chips are safe and durable. Biocompatibility, for instance, is a huge challenge, because tissue in the brain, can react badly to an implant, killing off the very neurons that the electronics are trying to connect to. Recent efforts suggest that a coating of growth hormones might mitigate this problem, while others have shown that chips, which slowly exude stem cells might also work.
Then there is the problem of powering 'the devices. Most existing implants such as the cochlear implants, for instance - are connected to a battery outside the head that can be replaced regularly. The electrodes in the spine and limbs could be powered this way, but it is less practical for a chip deep within the skull. Instead, such chips will need to be recharged by electromagnetic fields generated by a device outside the head, so power consumption will have to be minimal.
One solution might be to offload the more difficult processing to a portable computer outside the body, before passing the information back to the chips that stimulate the nervous system. In this way, Reid Harrison at the University of Utah in Salt Lake City has produced a neural chip that uses just eight milliwatts. That is less than the ÒstandbyÓ LED on the front of a TV set.
All the pieces are gradually coming together, but whatever happens, it will be a long time before these chips can become a mainstream treatment: the US Food and Drug Administration requires as much as 10 years of animal testing before a chip can be deemed safe enough to be implanted in human brains. That means the latest technology, such as chips that stimulate tactile sensations in the brain, will need extensive testing before clinical trials can begin.
Yet, even after the technology has proven itself, the social issues surrounding the treatment will need to be solved. Like the question of security, for instance. Last year, a team of researchers successfully hacked into a heart pacemaker and defibrillator through the wireless communication that allows doctors to adjust its performance. Although the device was not implanted in anyone at the time, it raised the possibility that hackers could disrupt a patient's treatment.
To make matters worse, there is currently no obvious way of protecting a defibrillator or pacemaker from a hacker without inhibiting a doctor from accessing it during an emergency. Since neural prostheses will rely so heavily on wireless links to communicate between the different components, the risk to these chips' may be even greater.
Nevertheless, paralysis is not the only condition that can be treated with chips in the brain. Others include:
- Deafness: The cochlear 'implant has been commercially available for many years. It detects sound and creates a signal that is fed directly into the auditory nerve. In this way, damaged portions of the ear can be bypassed entirely;
- Blindness: Retinal prostheses are being tested in blind people who lack the ability to turn light signals into neural signals. They can be plugged into the brain either at the retina itself, the optic nerve, or even the visual cortex;
- Parkinson's disease: Some people with Parkinson's are implanted with deep brain stimulation systems that can prevent some of the shaking that is characteristic of the disease. Though the surgery carries risks, a new study shows that people gained more than 4.5 ÒgoodÓ hours a day using the devices; and,
- Epilepsy: Devices known by some Òas Òbrain pacemakersÓ send regular electrical pulses to parts of the brain associated with the condition, helping to preventÓ the neurons from firing in the patterns associated with seizures.
Concluded.
- Sam Okoye, a retired Professor of Astrophysics and Life Fellow of the Nigerian Academy of Science, writes from London.
