Slight declines in hearing, smaller than the usual cutoff for diagnosing hearing loss, are associated with measurable mental decline in seniors, a new study suggests.

When researchers used a stricter threshold to include mild hearing loss, they found evidence that the well-established link between age-related hearing loss and cognitive decline might begin sooner than is recognized, according to the report in JAMA Otolaryngology-Head & Neck Surgery.

The seniors who had hearing problems at the more sensitive threshold would have been considered to have normal hearing by the current standard for diagnosing hearing loss: 25 decibels, the researchers note. But when the threshold was set at a hearing decline of just 15 decibels, which is comparable to the volume of a whisper or rustling leaves, some of the seniors had trouble hearing.

These people also had “clinically meaningful” cognitive decline, the study team found.

Some scientists suspect that hearing issues might lead to thinking problems because the brain has to redirect so much attention to hearing that it doesn’t get to exercise other mental functions as much.

“People with worse hearing use so much more brainpower to decode the words that are said, they don’t get to process the meaning of what was said, which is the intellectually stimulating part,” said the study’s lead author, Dr. Justin Golub, an assistant professor in the department of otolaryngology-head and neck surgery at NewYork-Presbyterian/Columbia University Irving Medical Center in New York City.

Golub compares brain fitness to physical fitness. If runners had to think about how to take each step, they wouldn’t get very fast, he explained. Similarly, parts of the brain involved in complex thinking don’t get as much “exercise” when more resources are directed to decoding the words in a conversation.

Beyond that, it’s been shown that “people with worse hearing socialize less — because it’s hard — and thus have fewer intellectually stimulating conversations,” Golub said. “The brain is like a tool that has to be maintained.”

For the new study, Golub and his colleagues analyzed information from the Hispanic Community Health Study (HCHS) and the National Health and Nutrition Study (NHANES), both of which contained data on participants who were given both hearing and cognitive testing.

The researchers focused on participants in the HCHS who were aged 50 or older and who had not developed early-onset hearing loss and those in the NHANES who were aged 60 to 69. That gave them a total of 6,451 people in the analysis, with an average age of just over 59 years.

After accounting for demographic and cardiovascular risk factors — both of which might impact the likelihood of developing cognitive problems — the researchers determined that decreased hearing ability was associated with worse performance on the cognitive tests.

“People who had difficultly hearing a whisper (but technically still had normal hearing), scored 6 points worse on a test of speed and attention than people who had absolutely perfect hearing,” Golub said in an email. “This took into account other factors, such as age. Scientists say that the 6-point change could make a meaningful difference in day to day function.”

The study wasn’t designed to examine how hearing loss might directly influence cognitive decline, the researchers acknowledge.

Still, Golub suspects that people might be able to remain more mentally sharp if they started wearing hearing aids as soon as they started to have even mild issues with hearing.

In fact, he said, “we’re running a randomized controlled trial right now treating one group of people with hearing loss and comparing them to a group with no treatment. We’ll see if in a couple of years the people with hearing aids are cognitively sharper.”