“These people are sensing something, and there’s some biological signals that correlate,” Dr. Petersen said. “I think it’s real.”

Experts emphasize that many people with such complaints will not develop dementia. Some memory decline reflects normal aging, they say, and some concerns reflect psychological angst. People who forget what they wanted in the kitchen or the names of relatively unfamiliar people are probably aging normally. People who forget important details of recent events, get lost in familiar places or lose track of book or television plots may not be, especially if they have more problems than others their age.

And much remains unknown about subjective concerns. In some studies, like Dr. Amariglio’s, highly educated people noticed changes more readily, but in other studies, less educated people did. Some studies suggest people who worry more about memory deficits have more dementia risk, but it is unclear if the worry reflects the changes they sense or if worrying itself increases risk. People with family histories of dementia could be reporting problems simply because they know about the disease and its genetic component. And, while a study presented Wednesday found slight correlations between subjective concerns and the highest-risk genetic mutation, ApoE4, that relationship remains unclear.

Experts are not yet suggesting doctors regularly screen people for “subjective cognitive decline” because much more research is needed and no effective dementia treatment now exists.

Dr. Richard Caselli, a neurology professor at the Mayo Clinic in Arizona, said that when patients cited cognitive problems, he ruled out “reversible things,” but did not recommend testing for Alzheimer’s because “if we do a scan and say, ‘Hey, we found some amyloid in your brain,’ there’s really nothing you can do.”

But subjective screening has value now for clinical trials, experts say, because it can help pinpoint people at higher risk for dementia to better determine if treatments can delay or prevent Alzheimer’s.