Doctors have made an accidental breakthrough that may unlock how memory works, during experimental brain surgery to control the appetite of an obese man.

The memory stimulation, which could potentially pave the way to treat disorders such as Alzheimer's disease, was an unanticipated side-effect of the treatment for obesity.

Electrodes were pushed deep into the man's brain, in this case to an area called the hypothalmus, thought to control appetite, and stimulated with an electric current.

Instead of suppressing the 30-stone patient's hunger, the experimental technique of deep brain stimulation (DBS) unlocked vivid memories of an experience 30 years earlier.

The surprising result has raised the possibility of a 'pacemaker' for the brain and a pilot study for patients with early Alzheimer's disease is now under way.

Lead researcher Professor Andres Lozano, of the Toronto Western Hospital, said: "This is a single case that was totally unexpected.

He said that once his team planted the electrodes, memory activity was most intense near a structure called the fornix, a grouping of fibres that carries signals within the limbic system, which is involved in memory and emotions and is situated next to the hypothalmus.

"To our surprise, as we turned on the device, he had a vivid experience of 30 years earlier,'' Professor Lozano said.

The 50-year-old man recalled in detail being in a park with friends and his then girlfriend when he was aged around 20. As the electric pulses were intensified, so the scene became more detailed.

Following surgery, the patient recovered for two months. When the electrodes were fitted a second time, more tests showed his ability to learn was dramatically improved when the current was switched on and his brain stimulated.

A year later he again performed well in memory tests when the electrodes were stimulated but less well when they were switched off.

"We knew immediately this was important. We are sufficiently intrigued to see if this could help people with memory disorders," said Professor Lozano. "We know very little about the circuitry of memory. This might give us some insight.''

The findings, reported today in the Annals of Neurology, the journal of the American Neurological Association, could offer hope to sufferers from the degenerative condition, which affects 450,000 people in the UK. However, experts warned that research is in the very early stages.

Professor Lozano, a professor of neurosurgery at the University of Toronto and a world authority on DPS said the approach could only address the symptom of forgetfulness. It would not modify the course of the disease but could still prove a useful tool in the fight against dementia.

"It gives us insight into which brain structures are involved in memory" Professor Lozano said. "It gives us a means of intervening in the way we have already done in Parkinson's and for mood disorders such as depression, and it may have therapeutic benefit in people with memory problems."

The technique is currently being used on six Alzheimer's patients as part of an initial pilot study. Three have so far had the £25,000-£30,000 device surgically implanted.

Rebecca Wood, Chief Executive of the Alzheimer's Research Trust, gave a cautious welcome to the findings, but said further work was clearly needed. She said: "It will be interesting to see whether this method offers any benefit to people with Alzheimer's.

"With the number of people with Alzheimer's forecast to double within a generation, we urgently need to find ways to tackle this awful disease, but research is hugely under-funded."

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