Forty years ago, the first bionic human featured in the hit US TV science-fiction series The Six Million Dollar Man. Today, he has become science fact. New technologies that "intervene" in the brain, building superhuman capabilities and enabling users to operate weapons or wheelchairs with the power of thought alone, are on the market or under development.
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Electrodes implanted deep in patient's brains have been shown to stabilise the shaky movements of Parkinson's disease and "creativity caps" that deliver magnetic pulses to the head are in use to boost memory and mathematical ability.
Scientists have demonstrated how a person in New York with a device implanted in their nervous system can control a robotic arm in the UK, moving it around and sensing the position of objects just by thinking about it.
But the rapid advance of the research is raising concerns that meddling with the brain could change people's personalities, create bionic supermen for military applications or be used to control minds with disturbing implications for society.
Today the Nuffield Council for Bioethics launches a consultation on the ethics of the new technolgies, the global market for which it says is worth $8bn and "growing fast."
"Intervening in the brain has always raised hopes and fears. Hopes of curing terrible diseases and fears about trying to enhance human capability beyond what is normally possible," said Thomas Baldwin, chair of the study and professor of philosophy at the University of York. "This challenges us to think what makes us human and why we think and behave in the way we do."
The most advanced technology is deep brain stimulation – the implanting of electrodes in the brain – which has shown dramatic results in improving movement control in Parkinson's disease. But some patients have developed severe side effects including personality changes, increased sexual urges and criminal behaviour. One study found half of those treated reported a deterioration in their marriage or relationships. "If that is replicated in further studies that will be alarming," said Professor Baldwin.
An electric coil worn in a cap or attached to the head with a band which delivers magnetic pulses to the brain has been shown to relieve the symptoms of severe depression in patients and boost mental performance in young adults. Known as the "creativity cap" and employing a technology called transcranial magnetic stimulation, it is available from online retailers. It suppresses some brain activity enabling the individual to focus on a particular task.
Alena Buyx, of the Nuffield Council, said: "A trial in the UK showed it improved performance in maths and there have been calls for it to be introduced for children in education. We know of children prescribed ritalin [a drug for attention deficit hyperactivity disorder] to boost their school performance. Should we try to create individuals with superhuman abilities?"
In Scotland, doctors have launched the first trial in the world of neural stem cells injected into the brain to replace damaged brain cells as a treatment for strokes. But there are fears the therapy could lead to brain tumours or changes in mood, behaviour and ability.
"Do these technologies alter our notions of personal responsibility? If someone is caught shoplifting, can they claim their brain implant made them do it?" said Dr Buyx.
Kevin Warwick, professor of cybernetics at the University of Reading, who has used himself as a guinea pig for implants in his nervous system with which he has controlled remote devices, said: "Military applications are being tested but are not yet in use. They involve remote control of vehicles and weaponry with the soldier in a safe location and the weapon in the battlefield. It blurs the distinction between man and machine. Who is making the decisions, who is responsible?"
The development of the technologies for use in warfare might be troubling for some. But the potential to help sufferers from diseases, including Alzheimer's and other brain disorders, is huge. "This is a big problem area and there is the possibility of helping an enormous number of people in different ways," Professor Warwick said.
Case study: 'Spike' in the brain helped to beat paralysis
Matthew Nagle was paralysed from the neck down after a knife attack severed his spinal cord.
The 21-year-old from Massachusetts in the US was left unable to move or breathe on his own. But in 2004, three years after the attack, he had an electrode array implanted in the surface of his brain – with each electrode "spike" penetrating 1.5 millimetres below the surface.
The implant enabled him to pick up objects, open emails, change television channels and play computer games. The electrodes were linked to the outside of his skull, where they were connected to a computer which was programmed to recognise his thought patterns and translate them into movements he was trying to achieve.
He operated a cursor on the computer screen and succeeded in moving it to switch buttons on and off. He was also able to control a prosthetic arm. The results were published in the scientific journal Nature.
The implant was removed after a year, in accordance with the protocol for the trial. Matthew Nagle died in 2007.
A second patient, aged 55, who had been paralysed since 1999, also had the implant but was less successful at operating devices remotely.
At least a dozen companies in the US are working on brain-computer interfaces, many for the US military.
Although there are hopes the technology could help people whose brains are damaged by illness or injury, there are also fears implants might be used to control challenging behaviour in patients with Alzheimer's disease or mental problems, by inhibiting antisocial tendencies and programming in "acceptable" responses.