Men who commit apparently motiveless murders in cold blood may be suffering from a previously unknown type of brain seizure at the time of the act, according to a psychiatrist from Harvard University.

Dr Anneliese Pontius believes she has discovered a completely new class of murderer, among the hundreds she has examined in secure mental hospitals in the United States.

More than 1 per cent of all male murderers may have committed their crime while in the throws of a "limbic storm", unleashed by some apparently innocuous memory which the hapless victim unwittingly triggered, she believes.

The limbic system of the brain controls emotion and motivation. It is an early development in evolution, present in many animals, but in humans it is kept in balance with the frontal cortex, a later less "primitive" system. Dr Pontius believes that 14 murderers she has examined committed their crimes as a result of a "Limbic Psychotic Trigger Reaction".

In one case, a man tipped a fisherman into the river and drowned him, because the angler reminded him of his dead father. The father had also been an angler and his photograph, displayed in the familyhome, had shown him in the position taken by the victim. It was this similarity which triggered the limbic storm, Dr Pontius says.

In another, fortunately non-murderous case, a monk had robbed a bank, and then gone to a strip club where he fondled the dancers. He suffered delusions, telling the women that he was a painter like Rembrandt.

When the seizure left him, the monk went to the police to confess and had difficulty convincing them of what he had done. His seizure was triggered by discovering he had no money in his pocket. This brought back memories of childhood poverty.

All the men suffering from this syndrome are shy loners, who ruminate on their hurtsrather than talking about them to friends and, eventually, forgetting them, Dr Pontius said.

A feature of their crime is that it is apparently emotionless and there is no motive and no planning. They are fully conscious of what they are doing,yet are horrified by the memory, Dr Pontius said.