Functional brain imaging allows us to look inside the living, working brain and see how it constructs our perceptions, emotions and thoughts. In most people the patterns of electrical activity that give rise to each mental state are similar; the brains of psychopaths and people with severe personality disorders, such as Stone, show distinct abnormalities.
Two brain areas have been found to be dysfunctional in antisocial people. One is the amygdala, a tiny nugget of tissue in the unconscious brain that generates feelings of alarm and emotion. The other is the frontal lobe, where self-consciousness emerges, emotions are registered and plans are constructed. It is also - crucially - the part that gives us control over our impulses.
In normal people the amygdala flares up whenever the brain senses anything that has emotional significance. It is especially sensitive to signs of emotion in others; a baby's cry, for example, or someone glaring at you. The alarm signals go first to another unconscious brain module, the hypothalamus, which creates an instant "fight or flight" response of the body, such as a pumping heart and tense muscles. The initial reaction is unconscious, but after a few milliseconds the alarm signals reach and trigger activity in the frontal cortex. This, finally, brings about the conscious feeling we think of as emotion.
Scans of psychopaths show a different pattern. When confronted with a potentially emotional stimulus the front part of a psychopath's brain fails to respond. In some cases this is because the amygdala does not "light up". In others it is because the signals from the amygdala fail to reach the cortex.
This unusual brain pattern makes sense of the cruel and manipulative behaviour typical of psychopaths. In most of us the amygdala puts an automatic curb on our selfishness because - by responding to others' distress - it makes us feel uncomfortable if we hurt someone. Psychopaths are not inhibited by their feelings, so the only thing that stops them damaging people is the calculation that to do so would somehow be detrimental to themselves.
This does not, however, make every psychopath a criminal. Most of them realise that their interests are best served by sticking to the right side of the law, and their emotional impoverishment often makes them spectacularly successful. One survey found that some 6 per cent of British managers fulfil the diagnostic criteria for psychopathy, a percentage six times higher than the proportion of psychopaths found in the general population.
The brain abnormalities that bring about apparently mindless violence are quite different from those giving rise to the calculating cruelty typical of psychopaths. Here it is not lack of emotion that is to blame, but uncontrolled emotion.
When signals from the amygdala reach the frontal cortex two distinct areas are triggered into action. One is the ventromedial cortex - the brain area behind the bridge of the nose. This makes emotion conscious. The other is the orbito-frontal cortex - the bit behind the eyebrows. The function of this area seems to be to inhibit emotional impulses.
The bodily reactions produced by the amygdala - flight, grovelling appeasement or aggression - start to happen before we are conscious of them, but the orbito-frontal cortex stops them being carried through - which presumably gives the conscious brain a chance to come up with a more sophisticated response.
In most mature people this frontal cortex control mechanism is efficient enough to prevent catastrophic impulsiveness, however strong the emotional stimulus. But in some people the frontal lobes are not active enough to override the urges pumping up from below. Children are like this: toddlers grab things they like and bash things they don't like because the nervous tissue in their frontal lobes (unlike that in their emotional brains) is not yet sheathed with myelin, the protein insulation needed to bring it on line.
Brain scans of violent criminals suggest that, in some ways, their brains are like those of infants. A study of 41 convicted murderers on America's Death Row showed that the majority had abnormal activity in the frontal lobes of their brains. Such studies are highly contentious and no one has yet convinced a jury that such abnormality is the prime cause of murder, but evidence is mounting to suggest it plays an important part. Certainly frontal-lobe dysfunction is known to produce a type of compulsiveness, known as "environmental dependency syndrome", in which people seem compelled to carry out whatever act they are "cued" to do. This often translates into antisocial behaviour: if such a person sees an attractive object they will automatically grasp it; an unlocked car, for them, may be seen as "asking" to be driven away.
To say that antisocial acts are due to a neurological fault is not to deny that a person's background and environment play a role. The modules in our brains, like the muscles in our bodies, will get bigger and stronger if constantly exercised, and if neglected they will atrophy. Musicians, for example, develop up to 25 per cent more tissue than other people in the cerebral area that processes music, and children who never hear language lose the ability to speak it normally, because the brain area that normally processes words does not get a chance to develop. The modules that grant us our most precious qualities - sympathy, self-control, a sense of morality - are no different in this respect from those that deal with relatively mechanical mental functions.
People who turn into monsters may begin with a relatively small neurological fault that is amplified into something disastrous by a malign environment. In some, however, the brain fault seems to be the most important factor: one study of 38 men and women charged with murder found that 26 came from "good" homes, and their crimes were inexplicable in conventional sociological terms. Their brain scans, however, showed they had 15 per cent less activity in the orbito-frontal cortex than the 12 whose backgrounds had clearly contributed to their behaviour.
Are such people insane? Legally, no. Frontal-lobe and amygdala dysfunction may bring about terrible behaviour, but does not cause a loss of contact with reality, and that is the current measure of madness.
This however, does not necessarily mean that the condition is untreatable - as psychiatrists have suggested in the case of Michael Stone. Whether it is regarded as mad or bad, antisocial behaviour is clearly linked to various brain modules being over- or under-active. Depression and anxiety are caused by a similar systems fault. So are Parkinson's disease and Tourette's syndrome. All these conditions have at times been seen to be afflictions of the soul rather than organic diseases; their status changed rapidly, however, when effective treatments were discovered for them.
If the will (and money) were there to research the physical causes of antisocial behaviour in the same way that other conditions have been investigated, the evidence suggests that these, too, could be treated. Just as Prozac alters the neurotransmitter that turns brain activity on and off in the areas affected by depression, so drugs could be developed to stimulate the frontal lobes or turn down activity in the amygdala. Existing drugs might even work; people with antisocial personality disorders sometimes have chemical abnormalities similar to those seen in addiction and depression, so treatments for those disorders would be worth trying.
It may even be possible to change potentially dangerous brains by manipulating them directly. Neuronal tissue is exquisitely sensitive, and can be turned "on" or "off" by the lightest touch of an electrode. Electronic or magnetic pacemakers - similar to those already fitted in the brains of people with Parkinson's disease - could be fitted to regulate activity in particular areas.
The technology required is already here, and brain-mapping by functional imaging is fast providing a detailed chart of the brain to guide the hands of surgeons and inform psychopharmacologists. The big obstacle to effective treatment for people such as Stone is public prejudice. The butchery of mid-century frontal lobotomies, and a superstitious belief that the brain is an extension of some metaphysical entity rather than an organic machine, combine to give psychosurgery the biggest "yuk" factor of all.
The idea of tinkering with brains of people we deem "antisocial" is rejected because of its associations with brain-washing, eugenics and totalitarianism. Given this, it is hardly surprising that psychiatrists turn a blind eye to the Stones of this world. At the moment they are being damned for not treating these people - but they'll probably be doubly damned if they do.
`Mapping the Mind' by Rita Carter is published by Weidenfeld & Nicolson, price pounds 25Reuse content