Stressful experiences can sculpt the brain too. Deep below the cortex, two almond shaped parts of the brain - the amygdala - detect danger in a split second and take over command of the brain, forcing you into action in times of perceived danger. The amygdala is part of the brain's emotional system - the limbic system. When you hear, see, feel or smell something, this information passes both to the cortex and to the amygdala. In fact, it gets to the amygdala before it reaches other parts of the brain. And if what you sense has been linked with danger, the amygdala springs into action.
This is because the amygdala functions as a kind of storehouse of emotional memories, and these memories and reactions are physically embroidered into the brain's wiring. Such changes in the emotional parts of the brain can be very hard to reverse through new learning, which is why it can be hard to overcome phobias and other emotional disorders that are so common.
But they can be overcome through scientifically-proven psychological therapies such as behaviour therapy and cognitive therapy. One particularly disabling psychological condition is obsessive-compulsive disorder. People suffering from this can feel a compulsion to perform repetitive rituals, such as washing their hands many times a day; this is often accompanied by a fear of contamination from touching anything.
A familiar treatment for OCD involves exposing the person to the stimuli which trigger the rituals - for instance touching a "dirty" object. Although it is stressful at first, after a while it becomes less distressing and they discover a new freedom in their lives. This behaviour therapy has been shown to physically sculpt and change the brains of people with obsessive- compulsive disorder, and these brain changes go hand in hand with the improvements in their mental functioning and behaviour.
People given behaviour therapy showed a beneficially reduced activity in a part of the brain known as the caudate nucleus, which is closely involved in obsessive-compulsive disorder.
So therapy that involves no drugs or physical treatment can change your brain. This has only been shown directly in the treatment of obsessive- compulsive disorder, in the parts of the brain linked to this condition. It's very likely, however, that similar changes would follow successful psychological treatment of other emotional disorders.
Part of the stress reaction triggered by the amygdala and other brain regions is to release steroid stress hormones into the blood. While these are essential fuel for fast responding to emergencies, if stress is excessive or prolonged, stress-related changes in brain chemistry can be harmful to brain cells, particularly in a part of the brain responsible for learning and day-to-day memory - the hippocampus. We need the hippocampus to remember what we did this morning, to remember the name of a new business contact or to recall what we were told five minutes before.
Stress, if it is severe or prolonged, can cause the neurones in the hippocampus to shrink, though they usually spring back into shape once the stress is lifted. It is likely that the problems with memory and concentration that people often experience during periods of stress are partly caused by this temporary stress-sculpture of the brain.
In very severe cases of stress these brain changes can be permanent. Some survivors of firefights in the Vietnam War who suffered post-traumatic stress were found later to have shrunken hippocampuses and poor memory. People who have suffered repeated abuse as children can also suffer the same kind of brain change because of their traumatic experiences.
This damage to the brain in turn makes it harder to cope with new stresses and difficulties that life throws up. This creates more stress and may shrink further the trembling web of brain connections in the memory and other cognitive systems. For young children brought up in harsh, stressful environments, these corrosive effects of stress on the brain can sap their ability to learn and prevent their intelligence developing fully.
In Washington DC, for instance, 9 per cent of six and seven-year olds in one deprived neighbourhood had witnessed someone being shot. A further 13 per cent had seen someone being stabbed, while a staggering 16 per cent had seen a dead body on the street. And witnessing muggings was commonplace - a quarter of these little children had seen one taking place. Stress of this type is likely to have profound effects on the minds of these children, as well as physical effects on the brains of at least some of them.
One of the best antidotes to the corrosive effects of too much stress is love and affection. When young mice are regularly stroked gently on the back with a soft, dry paintbrush, beneficial changes in their brain chemistry result, of a type likely to promote positive cognitive and emotional development in important brain centres. It shouldn't be surprising, therefore, that children deprived of cuddles and a close emotional relationship to one or more adults, show brains and bodies that are stunted. A few years ago our TV screens showed harrowing images of Romanian orphanages, a legacy of the Ceauc- escu regime. The pictures of row upon row of dull- eyed orphans sitting in bare, dirty rooms still haunts many people. In the better Romanian orphanages, the children are reasonably well fed, clean and are allowed access to the fresh air. What they too often lack, however, is much more important, namely the brain-nurturing interchange with a loving adult who learns to understand and respond to them.
But it is not just in Romania that children can be deprived of such essential mind and brain sculpture. In the Eighties, a group of researchers studied children in London who had gone into institutional care before the age of two, and left again sometime over the next five years. Though the children were given very good physical care, as well as decent mental and verbal stimulation, what they didn't get was a close relationship with one or two adults. Rather, they were looked after by an army of different staff - 50 on average over the course of their stay.
It is, of course, impossible for 50 different people to form the close relationship that a young child needs. So, it is not surprising to find that, in one study, a third of women who had been in institutions as children were psychiatrically ill by the time they were adults. In contrast, only one in 20 women who hadn't been in institutions as children were psychiatrically ill.
And these adult women who had experienced such poor emotional care as children tended to become poor mothers, lacking in warmth, harsh with their children and showing inconsistency and poor control over them. Thus, before our eyes we see not only faulty mind sculpture within a generation, but also between different generations.
The news was not all bleak, however. Some of these women who had had terrible early experiences actually became good parents themselves, and these tended to be the ones who had met a partner they could count on to help them, and in whom they could confide.
In short, loving relationships can help break the transmission of faulty mind sculpture across generations, as well as help to protect against the corrosive effects of stress.
`Mind Sculpture: your brain's untapped potential' by Ian Robertson will be published this week by Bantam Press, pounds 13.99