Brains can now be manipulated to change people's behaviour, raising serious social issues, warns Rita Carter

Here is a story: a burglar has just robbed a house. As he walks away - his swag safely stashed in an innocent looking suitcase - he drops his glove. He fails to notice his loss, but at that moment a policeman rounds the corner, sees the glove fall and calls out: "Hey! Wait!". The burglar turns, sees the policeman - and runs away.

So why did the burglar run? To most of us it is obvious: the burglar thought the policeman knew what he had been up to and was about to arrest him rather than reunite him with his glove. But where does this intuitive knowledge of what was in the burglar's mind spring from?

There is a surprisingly literal answer to this: it comes from a nugget of brain tissue about an inch up and back from your left eye. Our ability to read others' minds - a talent known to psychologists as the theory of mind - is one of an increasing number of sophisticated mental processes that have recently been pinned down to a particular brain "module". Others include the ability to delay instant gratification in favour of a long-term goal, the generation of new ideas and the switching of attention from internal thoughts and sensations to the outside world. Even things like humour and religious transcendence have been associated with activity in specific, discrete clumps of cells.

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The mapping of these states of mind was done by functional brain imaging, a technique in which people's brains are scanned while they carry out various mental tasks. The machines used for this - positron emission tomography (PET) or functional magnetic resonance imaging (MRI) - show which bit of brain produces a particular thought, feeling or perception.

The biological mechanics revealed by these studies are producing amazing insights into the way we tick - and raising potentially explosive social and ethical implications. Chief among them is the prospect that once we have a detailed brain map it may be possible to mould our personalities, experience and behaviour in any way we want - a sort of cosmetic surgery for the soul.

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Such an idea sounds like science fiction, and those involved in brain research loathe talk of it. The genetic parallel - designer babies and so on - has haunted the work of the genome mappers, and brain-mappers could do without that sort of attention. But it is impossible to view brain research without becoming aware of its possible applications.

The brain is a relatively easy organ to manipulate because it is exquisitely sensitive. A touch from an electrode or the faint pull of a magnet alters the electrical currents that activate its neurons. Place a magnet over the brain area that controls finger movement, for example, and so long as it is in place the person will be unable to move that part of their body. Stimulation from an electrode in the area that deals with emotion produces intense feelings of anger or sadness and a tiny electrical buzz in the temporal cortex brings old memories to consciousness like vivid dreams.

Our ability to change perceptions, beliefs and opinions by tweaking specific brain areas in this way is limited only by our ignorance of where and how these things arise. Until recently it was assumed that our "higher" cognitive functions were produced by the whole brain and that only relatively mechanistic abilities such as movement and sensory perception could be pinned down to a particular physical area. Functional brain mapping, however, shows that even the most sophisticated human attributes are largely localised, and as such they may be as easy to manipulate by direct brain stimulation as the movement of a finger.

Direct brain manipulation is likely to be used initially as therapy for mental illnesses. Certain psychological conditions show up on functional brain scans almost as clearly as broken bones on X-rays. In depression, for example, the area in the frontal lobes that is known to be holding old, bad memories in mind can be seen to be overactive, which is why negative thoughts continuously intrude into consciousness. At the same time the areas concerned with planning, thinking, and perceiving the outside world are unusually underactive, suggesting that the visual cortex of a depressed person does not register the same amount of light as that of a normal person. Their complaint that life seems "dull" and "gloomy" is literally true. Almost exactly the opposite pattern is seen in people who are maniacally high.

At present depression is treated by drugs like Prozac and by talking therapies. Both work by turning on the underactive areas in the brain and turning down the overactive ones. But their action is indirect - mediated by neurotransmitters in the case of drugs and time-consuming social interaction in the case of talking therapy. It is also slow and imprecise - the side-effects from drugs, for example, arise because the neurotransmitters affected by them activate (or inhibit) many brain areas other than those affected by depression.

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Brain-mapping is providing the information required to manipulate brain function without blitzing the nervous system with chemicals. Work on it has already begun - researchers have experimented with putting tiny magnets on the skull of depressed people for a few hours each day, just over the brain areas that need to be altered. Initial results look good.

Similar experiments have been carried out on people with obsessive compulsive disorder. People with this condition are driven to repeat seemingly pointless activities such as hand-washing or checking locks. Imaging studies show that their behaviour is driven by overactivity in part of the brain that prompts primitive instinctive behaviours such as grooming or scanning the territory for aggressors. Again, the magnets seem to switch off this frantic neural firing and restore a feeling of calmness.

So far depression and obsessive compulsive disorder are the only conditions for which direct brain manipulation has been tried, but in theory it could be used to alter any pattern of neural activity and thus to change any type of behaviour - including the violent, criminal or psychopathic.

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Much socially aberrant behaviour may be due to fairly easily identified brain dysfunction. Certain parts of the frontal lobes, for example, are responsible for inhibiting reflex actions such as grasping an attractive object, hitting out at an irritant or fleeing from a frightening situation. The reflex urges come from the limbic system and, if a person's frontal lobes are not working, they have no way of controlling them. The French neurologist Francois L'Hermitte has investigated a number of people with damaged or partially missing frontal lobes and all of them, he reports, have one thing in common: when confronted by some cue that suggests they should do something, they seem to be compelled to go ahead and do it. In many of them this shows up as a compulsion to steal: if a purse is left lying unguarded or a car unlocked, it is, to them, just demanding to be taken.

The inability to control impulses may also be a factor in many acts of violence. Brain scans of convicted murderers suggest that most of them have abnormal activity in their frontal lobes. In some this has been linked to traumatic head injury, in others to changes brought about by a lifetime of negative social and environmental influences. Whatever the root cause, the end effect is a disordered brain - a physical disease for which the patient is not to blame. This being so it seems pointless to punish - why not just fix their brains?

Why not, for that matter, "improve" the neural patterns in a lazy brain or tweak the activity in a slightly dull one? Why settle for a boring personality when you could jazz up your brain function and become instantly more creative or interestingly eccentric? Why suffer misery when you could turn off negative emotions and allow only the good ones to reach consciousness? Given enough knowledge about the way the brain constructs perceptions it might even be possible to produce in our heads a tailor-made virtual reality that will make interaction with the messy outside world almost entirely unnecessary.

These things are as certain to come as human cloning and, yes, designer babies. Like the issues raised by genetics, those arising from brain research need to be recognised and debated before the technology outpaces our understanding of how to use it. This science is too important to be left to scientists.

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Rita Carter is a writer specialising in medical subjects. Her book Mapping the Mind will be published by Weidenfeld and Nicolson later this month, price Pounds 25.00.