WASHINGTON (Reuters) - Scientists may have learned why some people retain sharp minds and clear memories despite having the so-called brain plaques and tangles that are the hallmarks of Alzheimer's disease.

In comparing the brains of these people to others who had all the memory-robbing symptoms of Alzheimer's, the researchers said on Tuesday they found those who avoided dementia consistently had a larger part of the brain called the hippocampus.

It is a structure vital to memory formation located in the brain's temporal lobe. Everyone has two of them -- one of the left side and one on the right side of the brain.

Dr. Deniz Erten-Lyons of Oregon Health and Science University in Portland, who led the study, presented at a meeting of the American Academy of Neurology in Chicago, said the findings could inspire new ideas for combating Alzheimer's disease, the most common form of dementia in the elderly.

The brain tissue of people with Alzheimer's disease contains abnormal clumps called amyloid plaques and irregular knots of fibers called neurofibrillary tangles.

Doctors have struggled to understand why some people with these plaques and tangles never experience the loss of intellectual and social abilities caused by Alzheimer's.

"Right now, we focus a lot on amyloid plaques and these tangles in Alzheimer's disease," Erten-Lyons said in a telephone interview.

"And I think one of the important things of our study was emphasizing that there's other things that we need to focus on as well -- other mechanisms that we don't know and do play a role in Alzheimer's disease. That needs to be further investigated because these could create new targets for prevention and therapeutic strategy," Erten-Lyons added.

His team studied the brains of 12 people who had extensive plaques and tangles but retained a sharp mind and clear memory, comparing them to 24 others who had extensive plaques and tangles and also had the usual Alzheimer's symptoms.

Based on brain scans taken while they were still alive, the researchers found that the hippocampus in the clear-minded people was about 10 percent larger than in the other people. Their overall brain volume also was about 5 percent greater.

"The changes associated with Alzheimer's disease usually begin in the hippocampus," Erten-Lyons said. "That's where the brain shrinkage starts in Alzheimer's disease."

When comparing the two groups of people, the researchers did not see any educational, socioeconomic or other differences and the results did not differ based on age or sex.

"It could mean two things," Erten-Lyons said. "Either they started out with bigger brains and need more accumulation of tangles and plaques before they show symptoms. Or plaques and tangles in some people do not lead to brain cell damage and loss of brain cells, which is usually the way we're thinking about what happens in Alzheimer's disease."