'Generally, the memories that we trust are more likely to be correct than the ones we don't trust,' said Roberto Cabeza, a neuroscientist at Duke University in Durham, North Carolina.

'However, in some cases we can be completely confident in an event that never happened,' said Dr Cabeza, whose study appears in the Journal of Neuroscience.

He said memories can come from two sources: a part of the brain called the medial temporal lobe that focuses on the facts and details of a memory, or the frontal parietal network, which involves the overall gist or familiarity of a memory.

People tend to have a high degree of trust in memories in which they can recall a lot of detail. But it is possible have familiarity without recollection, Dr Cabeza said.

He and South Korean researcher Hongkeun Kim of Daegu University wanted to understand the mechanism behind false memories. They did brain scans on people while they were encoding and remembering a group of associated words, like the names of farm animals.

They used functional magnetic resonance imaging, or fMRI, which can show the brain's activity in real time. Then a standard trick was used to evoke false memories.

'They were told horse and cow, but not pig,' Dr Cabeza said.

People often would falsely remember the word pig, and when they did, they would draw this memory from the gist region of the brain, which would light up on brain scans.

People who were highly confident in memories that were true showed increased activity in the detail region on the brain.

'If when remembering the event, you retrieve the gist without the specific details, you can have a false memory and remember things that never happened,' he said.

He said the research may have implications for diagnosing Alzheimer's disease, in which both types of memory are impaired.

'Healthy aging tends to impair recollection, but not familiarity,' Dr Cabeza said. But he said patients with Alzheimer's disease tend to lose both types of memories equally.

'In principle, one could measure brain activity associated with these two forms of memory to help in the early diagnosis of Alzheimer's disease,' he said. -- REUTERS