Scientists have taken a step towards finding out why memory deteriorates with age.
Alzheimer's affects four million people worldwide
Researchers in the United States have found that memory loss may be linked to so-called brain tangles.
These occur when twisted fragments of proteins within nerve cells clog up the cells.
Large numbers of these tangles are already known to cause Alzheimer's disease.
This latest research, by Dr Angela Guillozet and colleagues at Northwestern University, indicates that these tangles also occur in people who do not have Alzheimer's.
They examined the brains of eight dead people. Three had suffered from mild cognitive impairment - a more severe form of memory loss than that associated with ageing but less severe than Alzheimer's. The remaining five had healthy brains.
The researchers found tangles in all of the brains. However, the number of tangles was higher in those with mild cognitive impairment.
The researchers also found a direct link between the number of tangles and scores from memory tests carried out on the eight people before they died.
The team also looked at levels of beta-amyloid in the brains.
This protein is also associated with Alzheimer's. It clumps together and forms "plaques", which kill brain cells.
Their tests showed that none of the brains had abnormally high levels of this protein. This suggests that it is not involved in memory loss in old age.
It also suggests that it may not be responsible for the severe memory loss suffered by many with Alzheimer's.
The findings come as a second study suggests that depression could be an early sign of the disease.
Researchers at Boston University believe many people with Alzheimer's may also have a history of suffering from depression.
That theory is based on a study of almost 4,000 people, half of whom had the disease.
It showed that people with Alzheimer's were twice as likely to have suffered from depression when they were younger.
Overall, 14% of those with Alzheimer's had a history of depression compared with just 7% of those who did not have the disease.
The researchers were unable to explain why depression could be linked to Alzheimer's.
But Dr Robert Green, who led the study, suggested depression could adversely affect the brain or could be a very early symptom of the disease.
Many of those involved in the study had first become depressed more than 20 years before they were diagnosed with Alzheimer's.
Dr Green acknowledged that much more work was needed to examine this potential link.
"The answer remains unclear," he said.
But he added: "We are moving closer to developing accurate models for who will
be at highest risk for (Alzheimer's)."
Both studies are published in the Archives of Neurology.