In the study, researchers examined the brains of eight people who had died at ages 90 and older from various causes and found that some of them had signs of Alzheimer`s disease. However, tests of their cognitive function that were conducted shortly before their death showed that these people had memories that were as good as those of healthy people who were 30 to 40 years younger.

The results suggest that some individuals with Alzheimer`s disease may be protected against some of its symptoms, like memory problems, said lead study author Changiz Geula, a professor of cognitive neurology at Northwestern University Feinberg School of Medicine in Chicago, Illinois.

It is not clear why some people`s brains and memories seem to be protected against such symptoms, but the researchers suspect that genetic and environmental factors may be at work, Geula told Live Science.

The eight people in the study were initially part of a larger study of individuals who died in their 90s and whose cognitive function was examined shortly before their death. The study participants also agreed to donate their brains to science after their death.

Based on the results of the cognitive-function tests that were conducted among the larger group before death, the researchers selected and looked at the brains of eight people whose memories were as good as those of healthy 50- to 60-year-olds. They detected physical signs of Alzheimer`s disease, such as plaques and tangles, in three of those brains.

However, when the researchers examined the nerve cells in the hippocampus — a brain area that`s associated with forming memories — it turned out that, in the brains of the three people with the good memory abilities, the cells in this area looked relatively normal. This is unusual for people who have the plaques and tangles in the brain that signify Alzheimer`s disease, the researchers said. Usually, in people whose brains show evidence of plaques and tangles, the number of nerve cells in the hippocampus is reduced, Geula said.

This finding, which indicates that the nerve cells in the three people with superior memories have somehow been preserved, suggests that the preservation of these cells might be one of the factors that helps to protect these people`s memories, Geula said. However, its not clear that the preserved nerve cells were the cause of the people`s good memory abilities — it`s also possible that some other factor was responsible.

Moreover, if the preservation of these nerve cells is involved having a superior memory, it is likely not the only protective mechanism, he said. The researchers said that they think that certain genetic factors may also help to protect some people`s brains against the symptoms of Alzheimer`s disease.

It is also possible that a person`s diet or the amount of time he or she spends exercising may help to protect the brain against Alzheimer`s, although more research is needed before researchers can know whether this is true, Geula said.

Future research is also needed to confirm the new findings in a larger sample of people, he said.

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