Scientists have discovered how the brain can summon up our oldest memories from years past.
Dementia is linked to problems processing memories
They have found that the process is controlled by an area called the anterior cingulate.
It is hoped that the breakthrough could lead to the development of new treatments for Alzheimers and other forms of dementia.
The work, by the University of California Los Angeles, is published in Science.
Scientists have long known that a structure called the hippocampus processes recent memories.
However, it was also known that the hippocampus did not store this information permanently.
And just how the brain is able to retrieve more distant recollections, often from many years ago, had been a mystery.
The UCLA team engineered mice with a mutant form of a gene called kinase II, which eliminates the ability to recall old memories.
The animals were trained to recognise a cage, then tested for their memory of the cage at one, three, 18 and 36 days after training.
The mutant mice recognized the cage for up to three days after training - but showed no signs of recognition when tested again at a later date.
Earlier research suggested that the cortex - or outer layer of the brain - plays a role in the storage and retrieval of old memories.
Next, the UCLA team tested this theory by using imaging methods to track activity in a normal mouse's cortex during memory testing.
No part of the cortex lit up when the animal was exposed to the cage one day after training. When the mouse saw the cage 36 days after training, however, the images highlighted a part of the cortex called the anterior cingulate.
In contrast, the mutant mice's anterior cingulate never switched on during tests for distant memory.
In the third part of their experiment, the researchers injected normal mice with a drug that temporarily turned off the anterior cingulate.
This did not disrupt the animals' memory of the cage at one and three days after training - but did at 18 and 36 days.
Lead researcher Professor Alcino Silva said: "We now had several pieces of evidence all pointing to the same conclusion - the anterior cingulate plays a special role in keeping our early memories alive.
"Our work with the mutant mice also suggests that kinase II is critically involved in preserving our oldest memories."
Professor Silva believes the anterior cingulate assembles signals of an old memory from different sites in the brain.
Dementia may result from a malfunction in this assembling process, leaving the memory too fragmented to make proper sense.
Harriet Millward, of the Alzheimer's Research Trust, told BBC News Online: "Both ageing and certain aspects of Alzheimer's disease and other dementias - such as agitation, depression, apathy, and loss of attention - are all accompanied by reduced activity in the anterior cingulate.
"This study is interesting in making such a clear distinction between short-term and long-term memory and associating the anterior cingulate specifically with the latter.
"However, this has only been shown in mice so far. If these findings can be supported by studies in humans, then they will make a useful contribution to our understanding of Alzheimer's disease and other dementias."