Faces are hard to remember. Even harder are the names that go with them. It’s one of the most common problems people face as they get older.

In puzzling over why that is, Reisa Sperling and her colleagues at Harvard Medical School have discovered startling things about how memories are made and why people lose them, particularly those on the way to Alzheimer’s disease.

“Names and faces are so difficult to put together because there’s no obvious connection between them,” says Sperling, who is an associate professor of neurology. “There’s no ‘Bill look,’ or ‘Mary face.’”

Working at Brigham and Women’s and Massachusetts General, two Harvard-affiliated hospitals in Boston, she and her team did brain scans of 52 elders, who included normal individuals, those with mild memory problems, and those impaired with Alzheimer’s disease. While their brain activity was being scanned, these people tried to match unfamiliar faces with fictional names.

The results were surprising. The brain’s memory center, a tiny, sea horse-shaped bit of tissue deep in the brain, was more active in the mildly impaired individuals than in those with no obvious mental problems. Their brains seemed to need to work harder than normal to make new memories.