Overworked, underused

When the brain has to struggle to make sense of the world — because of vision or hearing problems — its cognitive ability declines, says Jane E. Brody 

Now and then I hear someone (myself included) proclaim "my brain is on overload." This is not surprising given the myriad complex tasks the brain performs, among them enabling you to learn, plan, remember, communicate, see, hear and smell.

Perhaps also not surprising, a growing number of studies have linked compromised sensory functions like poor vision and hearing to a decline in cognitive abilities. The brain, it seems, can do only so much, and when it must struggle to make sense of the world - from reading the words on a page to understanding the spoken word - it may be less able to perform other important tasks.

While a cause-and-effect relationship has yet to be established, evidence is gradually increasing to suggest that uncorrected deficits in vision and hearing can accelerate cognitive decline.

The latest study, published in JAMA Ophthalmology, found that among a representative sample of nearly 3,000 older Americans and a second sample of 30,000 Medicare beneficiaries, poor vision was associated with poor cognition. The two data sets used different measurements of cognitive abilities like memory, orientation and planning, and the consistency of their findings suggests that the association between vision impairment and compromised brain function is real, the researchers concluded.

The lead author, Dr Suzann Pershing, ophthalmologist at Stanford University School of Medicine, said "while this association doesn't prove vision loss causes cognitive decline, intuitively it makes sense that the less engaged people are with the world, the less stimulation they receive, and the more likely their cognitive function will decline."

An earlier study at the University of Michigan found that those with poor vision had a 63 per cent greater risk of developing dementia. Those with poor vision who did not visit an ophthalmologist were five times more likely to experience cognitive decline and nine times more likely to develop Alzheimer's.

"This is not necessarily a one-way relationship," Pershing said. "If you improve the vision of people with cognitive impairment, they can function better."

Likewise with hearing, where there is stronger evidence that hearing loss can speed cognitive decline. However, it is not yet known whether wearing properly adjusted hearing aids can decrease the risk of dementia or slow its onset. A study sponsored by the National Institute on Aging should show in five years whether using hearing aids can help preserve brain function in people with hearing loss as they age.

Dr Frank R. Lin, otolaryngologist at the Johns Hopkins Center on Aging and Health, said "the potential impact is huge," especially now. Companies like Apple and Samsung are expected to soon market hearing aids that cost perhaps $50 to $300 instead of $5,000 a pair, making it accessible to many more people.

Meanwhile, here's what we already know. A prospective study of 1,984 older adults directed by Lin found that those who initially had hearing loss were 24 per cent more likely than their age-mates with normal hearing to experience cognitive decline within six years; their cognitive abilities declined up to 40 per cent faster. They had greater problems with brain functions like thinking and memory, developing them on average three years earlier than people their age with normal hearing. And the more severe their hearing loss, the greater their cognitive loss. "Older adults with hearing loss face an increased risk of dementia even when you control for diseases like diabetes and high blood pressure," Lin said in an interview. "So we think they're causally related."

He suggested three ways that may explain how poor hearing and dementia are linked. One involves "cognitive load" - when you can't hear well, the brain receives garbled signals, forcing it to work harder to derive meaning from the message. Another mechanism may be that people who can't hear well tend to become socially isolated, which results in diminished cognitive stimulation and cognitive loss. Perhaps most important is a third possible explanation involving brain structure; hearing loss results in a faster rate of brain atrophy mostly over the hearing portion of the brain, which is also involved in memory, learning and thinking.


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