Scientists are to investigate whether a lack of hearing aids is helping to fuel the Alzheimer's epidemic.
Even mild hearing loss is said to double the risk of dementia, and severe deafness increases it five-fold.
Hearing impairment has been found to raise the rate of cognitive decline with age by about 36%.
Experts believe deafness undermines working memory by making it harder to concentrate and think.
Studies also indicate that hearing loss actually changes brain structure, causing shrinkage in some regions most affected by Alzheimer's. In addition, the social isolation caused by deafness may speed up mental decline.
Because of this association, researchers in the US now plan to find out if better treatment for hearing loss can reduce rates of dementia.
If the connection between poor hearing aid provision and dementia is proved it will have far-reaching implications in the UK too. Here, a leading charity has mounted a major campaign against NHS plans to ration hearing aids.
Dr Frank Lin, associate professor of geriatric medicine at Johns Hopkins University in the US, who is leading the study, said: "Overall, I would say that the epidemiologic evidence is moderately strong that hearing loss is exerting an effect on the risk of dementia that is not solely being driven by a correlation from a common cause.
"The bigger and more critical question, however, is whether treating hearing loss could actually mitigate some of this risk irrespective of the mechanism. This is a question that we are just beginning to address now through the development of a definitive clinical trial which will determine whether treating hearing loss in older adults can reduce the risk of cognitive decline and dementia."
He said that until the trial was complete it was "impossible to say" that poor distribution of hearing aids was worsening the Alzheimer's epidemic, but added: "The possibility of even slightly reducing the proportion of risk associated with hearing loss is important given the high prevalence of hearing impairment in older people."
In the US, current regulations require patients to spend thousands of dollars over many months to be specially fitted for hearing aids, Dr Lin pointed out.
"Hearing aids are out of reach for the vast number of Americans," he said, speaking at the annual meeting of the American Association for the Advancement of Science (AAAS) in Washington DC.
The US government-funded study will recruit around 800 patients to see if hearing loss treatments can reduce brain decline.
A link between hearing impairment and dementia was first suspected in 1989 but until five years ago hard evidence was lacking.
Then in 2013 a team led by Dr Lin showed that hearing loss could accelerate the rate of cognitive decline by about 36%.
Like dementia, hearing loss mostly affects the elderly.
The number of people with hearing impairment doubles with every decade of age, so that almost two thirds of the over-70s suffer from significant hearing loss.
The proportion of those affected using hearing aids is very low - an estimated 25% in the US.
In the UK, the NHS says 6.7 million people could benefit from hearing aids, but only 1.4 million (around 20%) use them.
One in six people in the UK, or 11 million, has some form of hearing loss.
Paul Breckell, chief executive of the charity Action on Hearing Loss, said there was "a significant body of evidence" showing that unmanaged hearing loss can lead to dementia.
He added: "Evidence shows that mild hearing loss doubles the risk of dementia, with moderate hearing loss leading to three times the risk, and severe hearing loss five times the risk ..
"We are concerned that Clinical Commissioning Groups in England are considering cuts to hearing aid provision, with the North Staffordshire CCG the first to ration them since the inception of the NHS.
"As a charity we have successfully campaigned to stop other CCGs from following suit and we are calling on all CCGs to continue to provide hearing aids to all those who need them, particularly in light of the priority that NHS England has placed on hearing loss through its development of a Commissioning Framework for hearing loss services which will provide guidance for CCGs and is due to be published in April 2016.
"Hearing aids offer a lifeline to many, especially older people with hearing loss who would otherwise be sat at home alone unable to communicate with the outside world. They are a highly cost-effective intervention and are accepted to be the only viable treatment for people with adult-onset hearing loss."
Sue Archbold, chief executive of The Ear Foundation charity, said: "We need to ensure that the public are aware of the potentially damaging consequences of hearing loss and we must ensure that the rationing of hearing aids in some areas of NHS stops now to ensure that we don't add to the number of dementia cases leading to heartache for families and additional costs to the NHS.
"We know that hearing loss is linked with dementia - and we know that today's hearing technologies are hugely beneficial in addressing hearing loss.
"We need to increase access to hearing aids early through the introduction of an adult hearing screening programme, to ensure that everyone has the opportunity to hear and possibly delay dementia."