I have recently read a paper that, I believe, has not received the attention it deserves, at least in the media. It is a study of the re-organization of the brain in response to violence. What it is that suddenly triggers a violent behaviour in some individuals is not known with any certainty, but it has long been suspected that exposure to violence leads to increased violence. We have, after all, been repeatedly told that watching violent movies results in individuals who are more readily capable of violence. But why and how?

A paper by Chris Kelly and his colleagues at Columbia Univeristy has far-reaching implications in this regard. They showed volunteer subjects clips of violent films and measured activity in different areas of the brain, using brain imaging techniques. Their results show, in brief, that repeated exposure to violence reduces the intensity of activity in a specific region of the brain known as the lateral orbito-frontal cortex progressively. The implication here is that the more violent movies are watched, the greater the reduction in orbito-frontal activity. This in itself may suggest an adaptation that makes individuals more immune to violence, adaptation being a very common occurrence in the brain. But the study goes further, to show that connections between brain areas must be in a dynamic state, and hence modifiable by experience - in this case the experience of violence - even in adult life. For the orbito-frontal cortex is connected with another brain area - the amygdala - which is known to be responsive to fearful and aggressive faces. This connection seems to be critical in the control of aggressive behaviour. The strength of connections between the two brain areas diminished progressively as violent movies were watched, and led to more aggressive tendencies. The study also strongly suggested that there developed a progressive diminished control over the initiation of violent aggressive behaviour. For there was a concomitant increase in activity within areas known to be critical in motor planning (for aggressive behaviour, among others). This suggests that the connections between the orbito-frontal cortex and yet other regions of the cortex are also in a dynamic state.

It is important to note that the study exposed subjects to violent films over relatively brief periods, each clip viewed lasting seconds rather than hours, as is common for violent movies. Hence, it does not take prolonged exposure to alter strength of connections in the brain, and in this instance, to loosen the inhibition that leads to violent aggressive behaviour.

There is a lesson in this for society, and that is why I am surprised that so little has been made of it by the media. We have discussed endlessly whether the extent of violence shown in Western movies is not damaging our already violent societies. And yet here is a study, which shows that even brief exposure can alter the balance of connections in the brain and tip them in a direction that is not socially advantageous. And there is, perhaps, a problem for legislators too. What if someone can come armed with evidence derived from brain scans to show that the crimes that he or she has committed is the result of altered brain states, induced by watching violent movies? It would be a manner of shifting responsibility. I do not know enough about law to discuss this point, but I should not be at all surprised if defense of this kind surfaces in future hearings of criminal violence cases.

But there is, above all, a great deal of interest in learning that such complex behaviour is held in check by a fine system of balance through the interactions between brain areas and that this balance is so vulnerable - even over very brief periods and even in adult life - to environmental influences. There is much in this work that is of interest for future studies in the neurobiology of aggressive behaviour and violence.