Researchers have uncovered a new clue to the cause of Alzheimer’s disease.

The brains of people with the memory-robbing form of dementia are cluttered with a plaque made up of beta-amyloid, a sticky protein. But there long has been a question whether this is a cause of the disease or a side effect. Also involved are tangles of a protein called tau; some scientists suspect this is the cause.

Now, researchers have caused Alzheimer’s symptoms in rats by injecting them with one particular form of beta-amyloid. Injections with other forms of beta-amyloid did not cause illness, which may explain why some people have beta-amyloid plaque in their brains but do not show disease symptoms.

The findings by a team led by Dr. Ganesh M. Shankar and Dr. Dennis J. Selkoe of Harvard Medical School were reported in Sunday’s online edition of the journal Nature Medicine.

The researchers used extracts from the brains of people who donated their bodies to medicine.