Fear has been eliminated from the human mind for the first time in a series of pioneering experiments that could open the way to treating a range of phobias and anxiety disorders with behavioural therapy rather than drugs.

Scientists have selectively blocked thoughts of fear by interfering with the way memories are "reconsolidated" by the brain.

It could lead to new ways of treating the thousands of people whose lives are crippled by fear and anxiety relating to phobias and memories that go back many years.

The research, funded by the US National Institute of Mental Health, may offer an alternative form of treatment to the current use of drugs, which have side-effects.

The study suggests that it may be possible to permanently eradicate an overwhelming fear by relatively simple behavioural therapy.

"Previous attempts to disrupt fear memories have relied on pharmacological interventions. Our results suggest such invasive techniques may not be necessary. Using a more natural intervention ... allows a safe way to prevent the return of fear," said Elizabeth Phelps of New York University, who led the study published in the journal Nature.

Conventional behavioural therapy involves exposing people to a phobia - such as showing a spider to arachnophobes - under "safe" conditions.

The new research goes a step further by deliberately triggering a fear memory and then trying to interfere with the way it is restored or "reconsolidated" by the brain within the critical minutes or hours after the memory was revived.

Dr Phelps said that it was very similar to conventional treatments of phobias but the key difference was that the timing was critical.

"By paying attention to the way memories are stored and restored we can perhaps target the therapy by changing the timing of the interventions," she said.

The idea is not to create a new memory saying that the phobia in question is safe, but to retrieve the original memory and manipulate it when it is being restored, or reconsolidated, to show that it is no longer dangerous, the scientists explained.

"Our research suggests that during the lifetime of a memory there are windows of opportunity where it becomes susceptible to be permanently changed. But understanding the dynamics of memory we might, in the long run, open new avenues of treatment for disorders that involve abnormal emotional memories," said Daniela Schiller, the study's lead author and a post-doctoral fellow at New York.

The findings came out of previous work on laboratory rats showing that it was possible to eliminate the fear of a particular sound associated with an electric shock.

This could be done by "extinction training", in which the rats were exposed repeatedly to the tone without any electric shocks.

However, the timing of this training was crucial. Fear of the sound was only erased in those rats that were trained after an interval of a few minutes but no longer than a few hours after the fear memory was revived.

The latest study, on human volunteers given electric shocks when shown coloured cards, was based on the rat tests. Only those people whose retraining took place within a certain time window after a fear memory was revived showed signs of fear elimination.

A year after the experiments, 19 of the volunteers took part in further tests. Those who had received training more than six hours after the fear memory was revived still showed signs of fear towards the coloured cards a year later. Those who had been trained within the time window showed no signs of fear - indicating that the fear memory had been eradicated.

"Timing may have a more important role in the control of fear than previously appreciated. Our memory reflects our last retrieval of it rather than an exact account of the original event," Dr Phelps said.

"It means we can target the time window when we know memories are being reconsolidated. In terms of changes to behavioural therapy, it will be subtle changes in the way it's implemented but we hope that it will have dramatic results," she said.