Issue date: Monday 7 July 2003

'You don't know your own strength': scientific basis for escalation of physical conflict identified

Scientists have shown for the first time in the laboratory what many have suspected for years: physical force escalates dramatically in a tit-for-tat situation.

We've all heard children who have been fighting saying "they hit me harder". Wellcome Trust researchers have conducted an experiment showing that this perception really has a neurological basis. The research findings have implications for the understanding of schizophrenia, and possibly the slapping of children by parents.

The experiment, the results of which are published in the 11 July edition of Science magazine, involved six pairs of people engaged in a tit-for-tat situation.

A motor was used to apply a fixed force to the finger of the first participant, who was then told to apply what they thought was the same force to the other person's finger. Over eight turns, the force applied increased a massive 14-fold. The participants always responded with a force one third or one half greater than the force applied to them. However, this escalation only occurred when the force was applied directly with the finger and not when it was applied using a joystick.

"The results showed that to get same feeling of force, you need to exert more force," said Wellcome Trust researcher Dr Sukhwinder Singh Shergill. "So when you apply force yourself by pushing with your finger it feels less than when you use a machine. It is well known that a system in brain de-emphasises the effects of our own actions, but this is the first time it has been measured."

"It is not to do with aggression, but means we are less aware of what we do," said his colleague Professor Chris Frith.

Just before you make a movement you send signal to a specific brain region to warn it what to expect, Dr Shergill explained. "The altered activity in this particular sensory area means that you tend to apply more force than you think. This mechanism also explains why you cannot tickle yourself - the brain already knows what sensation to expect and alters the brain activity responsible for the sensation accordingly. But when someone else tickles you there is no chance to adjust your brain perception, and you feel the full effects."

This lack of accuracy in directly applied force may have implications for contact sports like boxing, where the force you think you are applying will actually be greater than you think. Similarly, "it may not be possible for parents to accurately judge the force they apply when they smack their children, and this experiment would suggest that they will smack harder than they think or intend," he added.

This brain mechanism may also allow you to distinguish the origin of the various forces normally perceived by your body into those that are due to your own actions and those that may be due to external causes, noted Shergill. It also answers the question - relevant to pickpocketing - was that sensation due to my own action of putting my hand there or did someone else do it?

This may be particularly relevant to people affected by schizophrenia who can have difficulty in distinguishing between these options. When they experience bodily sensations - such as when they move their own arm or speak - they mistakenly believe someone else is responsible for them.

It has never previously been clear why this is happening, said Shergill. "One theory suggests that these sensations occur in schizophrenics if the system for predicting the effects of our movement breaks down, and fails to moderate its activity to distinguish between a force produced by ourselves and one produced externally."

This may also underlie their experience of hearing voices, which are the result of inner speech which, without the signals that it has been self-generated, is experienced by schizophrenics as coming from someone else. Dr Shergill has recently used brain imaging to show that a related phenomenon occurs in schizophrenics with auditory hallucinations.

"We now hope to use imaging techniques to look at the brains of healthy subjects and those with schizophrenia, to identify which brain areas are responsible for this activity and which areas don't work if things go wrong. Eventually, it may be possible to use drugs or electrical stimulation to reactivate the region," he added.

For more information contact

Dr Sukhwinder Singh Shergill
Tel: 020 7848 0355

Shaun Griffin
Wellcome Trust Media Office
Tel: 020 7611 8866
Mobile: 07710 307059
E-mail: s.griffin@wellcome.ac.uk

Notes to Editors

• Dr Sukhwinder Singh Shergill works at both the Institute of Psychiatry at King's College London and the Wellcome Department of Imaging Neuroscience, Institute of Neurology, University College London. Professor Chris Frith, Paul M Bays and Professor Daniel M Wolpert, the paper's other authors are at the Institute of Neurology, University College London.

• The Wellcome Trust is an independent, research funding charity, established under the will of Sir Henry Wellcome in 1936. The Trust's mission is to foster and promote research with the aim of improving human and animal health. Website: www.wellcome.ac.uk