A toxic web of a protein linked to Alzheimer's could be the key factor that determines whether a person will lose their memory with age.

Researchers with Northwestern University's Feinberg School of Medicine compared the number of fiber-like tangles of a protein known as tau in the brains of individuals without Alzheimer's who had lived well into their 80s.

They learned the individuals who had a razor-sharp memory also showed lower levels of the tau protein than those with an average memory for their age.

Oddly, both groups showed similar levels of brain-coating beta-amyloid plaque, which is the other hallmark of Alzheimer's disease patients.

Before the study, it was widely assumed that tau protein tangles accumulated in the brain as a natural part of the aging process. Now researchers believe some individuals may be immune to these memory-impairing protein bundles, which cause cells in the brain to burst as they build up.

The scientists hope to one day find what makes these so-called super-aged individuals resistant to tangle formation.