Lead researcher Changiz Geula, a research professor of neurology, calls such elderly people with laser sharp memory "super aged".
He has revealed that the fibre-like tangles they observed consist of a protein called tau, which accumulates inside brain cells and is thought to eventually kill the cells.
He points out that such tangles are found in moderate numbers in the brains of elderly, and increase substantially in the brains of Alzheimer's disease patients.
"This new finding in super aged brains is very exciting. It was always assumed that the accumulation of these tangles is a progressive phenomenon through the aging process. But we are seeing that some individuals are immune to tangle formation and that the presence of these tangles seems to influence cognitive performance," said Geula.
For their study, the researchers examined the brains of five deceased people, who were considered super aged because of their high performance on memory tests when they were more than 80 years old. They compared the brains of such people with those of elderly, non-demented individuals.
While making a presentation at the Society for Neuroscience annual meeting in Washington, D.C., Geula revealed that the number of plaques in the brains of the super aged was similar to that in the brains of the normally aging group.
Just like tangles, plaques also are found in modest numbers in the brains of aged individuals, and show a dramatic increase in number in Alzheimer's disease.
Geula said that the lower number of tangles in the super aged appears to be the critical difference in maintaining memory skills.
He even revealed that some of the super aged in the study performed memory tasks at the level of people who were about 50 years old: after being told a story, they were able to remember it immediately after and still accurately recall its details 30 minutes later.
The subjects could also remember a list of 15 words and recall them equally well when tested after 30 minutes.
Geula revealed that his team's future studies would focus on why do cells in super aged brains become more resistant to tangle formation.
"We want to see what protects the brains of these individuals against the ravages that cause memory loss. Understanding the specific genetic and molecular characteristics of the brains that makes them resistant, someday may lead to the ability to protect average brains from memory loss," he said.
Geula's research is part of a larger super aging study at Northwestern's Cognitive Neurology and Alzheimer's Disease Center (CNADC).
A number of super aged individuals have been identified, and are being followed up annually with tests of cognitive abilities.