UNDER THE MICROSCOPE : Road rage: the inside story

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I am A neuroscientist. As such, I wonder what is actually happening in a brain - unabetted by psychoactive drugs - to transform an individual, at some hot and sudden moment on the road, from an everyday person into a monster. I want to know the road rage story from inside the skull. It is not, by anyone's standards, "logical" behaviour. Yet anyone who, until that moment, could not be distinguished from any other road user, cannot be a habitual psychopath. So what transforms a "normal" brain so radically and so quickly?

As long ago as 1949 an American neurologist, Paul MacLean, wished to explain seemingly mindless human behaviour: he turned to the unlikely and unglamorous starting point of the physical brain itself, with its intricate and enfolding anatomy, to develop an intriguing theory.

Our brains are a hotch-potch of evolution. Over the years, different parts have expanded or idled to accommodate our varying lifestyles within the animal kingdom. In the case of us humans, the most astonishing expansion - even outstripping our chimpanzee cousins - is the "cortex". The cortex (Latin for bark) encircles the rest of the brain, just as its arboreal namesake does a tree. This region has been linked with so-called "higher" functions, such as memory, sophisticated reasoning, language and abstract thought.

MacLean's great contribution was to point to the apparent physical disconnection between the cortex and the next layer down in the brain, the "limbic system". This is relatively well developed in all mammals, and has been associated with emotions. It is easy to guess, from this simplified neuroanatomy, where MacLean's theory was heading. He suggested that, while the cortex was prevalent most of the time, the less rational, "emotional" aspects of our mental functioning occurred when the limbic system escaped the sobering control of the reasoning cortex.

This fits happily with the idea of "bottling up" emotions, used on occasions to explain road rage. But surely there is a difference between "emotions" that may be excessive, but are still appropriate to the moment - such as weeping at an old movie, or declaring you are in love - and road rage. One of the most chilling features of road rage is that the aggression is out of context and out of conscious control.

Interestingly, MacLean also incorporated into his scheme the oldest part of the brain, its very core, around which the outer layers are built: the "brainstem". This is a common feature of most brains, even those of reptiles. MacLean reasoned that, if perchance the brainstem were left to its own devices, reptilian-type behaviour would ensue, characterised by stereotyped instinctive action.

Road rage is in many ways like MacLean's "reptilian" level, where there is neither the reasoning of the cortex nor even the subtle and sophisticated emotion of the limbic system. Instead, literally mindless raw aggression hurtles along a pre-programmed course, transcending the personality of the individual. The complex, interactive and multi-coloured behaviour that dignifies the human being is downgraded to an instinctive aggression more redolent of a snake.

Nowadays, neuroscientists find MacLean's anatomy too simple. It is impossible to assign different functions, such as emotion or memory, to discreet regions of the brain: they all act in parallel. Somewhere in the brain, though, a chemical match is struck in the summer heat and a neurophysiological forest fire ensues. The identification of either match or fire, in terms of actual cells and molecules, is infinitely more elusive and complex than MacLean thought. But a clue to understanding road rage may well lie among the grey folds and crevices, independent of "bottled up" memories, emotions and character - a sinister relic of our cerebral past.

! Susan Greenfield is a neuroscientist at the University of Oxford, and Gresham Professor of Physic, London.