The hippocampus is the magenta area at the base of this brain
Having a large hippocampus - a part of the brain involved with memory - seems to provide protection against the symptoms of dementia, a study suggests.
A US team compared the brains of 35 people who had Alzheimer's "plaques", some of whom died with sharp minds and others who showed no dementia symptoms.
The hippocampus, an area at the base of the brain, was on average 20% larger in those with cognitive functions intact.
The Alzheimer's Society cautioned that it was a "relatively small study".
The research was presented at a meeting of the American Academy of Neurology.
It has long been recognised that people can die with all the biological evidence of Alzheimer's - such as a build-up of plaques and tangles within the brain - but having remained perfectly lucid until the last.
Researchers at Oregon Health and Science University in Portland compared the brains of 12 such people with 23 others who had similar levels of plaques, but had been diagnosed with Alzheimer's before death.
The hippocampus is located close to the junction with the spinal cord and is believed to "encode" experiences so they can be stored as long-term memories in another part of the brain.
"This larger hippocampus may protect these people from the effects of Alzheimer's disease-related brain changes," said lead researcher Deniz Erten-Lyons.
"Hopefully this will lead us eventually to prevention strategies."
The Alzheimer's Society said it remained unclear from this "relatively small study" whether the larger hippocampus really was the reason why people with dementia did not display the signs.
"However this work is consistent with increasing research that has shown that people with higher levels of education or cognitive reserve may be protected from some of the effects of dementia," said Professor Clive Ballard.
"This is an exciting area of research which needs more exploration."