It is commonly known that many people experience hearing loss as they age. A lifetime of exposure to loud noises can lead to a condition known as presbycusis, which a gradually occurring loss of hearing as one grows older. According to the National Institute of Health, approximately one-third of American adults between the ages of 65 and 74 suffers from hearing loss while about half of adults over 75 struggle to hear, so there’s no doubt that presbycusis is a common affliction of the elderly.
While hearing loss is known to affect, well, one’s hearing, it’s not overwhelmingly clear how one’s age-related hearing loss might be linked to overall cognitive decline. There are, however, a number of theories connecting the two conditions. Here are a few:
Cognitive Load Theory
This theory posits that one’s cognitive load – or the amount of processing effort required for someone to perform a task – increases with conditions like hearing loss, which forces someone to exert more effort to understand things like a conversation. As more effort is needed for certain tasks, the brain has less cognitive energy left to process secondary tasks. Thus, cognitive overload caused by hearing loss or other sensory issues can cause negative structural changes within the brain and, ultimately, cognitive decline.
Researchers think that the Cognitive Load Theory might provide a good explanation as to why hearing aids and cochlear implants can increase one’s working memory and ability to perform executive functions like self-control and time management. That being said, there isn’t much empirical evidence to support the notion that increased cognitive load causes cognitive decline.
Common Cause Theory
The Common Cause Theory argues that there is some age-related common factor that causes both cognitive and sensory decline. While no one factor has been identified yet, some possible factors include genetics, neural degeneration, cardiovascular or cerebrovascular diseases, or general physical health. These common causes would have to affect both sensory and cognitive functions, which means that treatments that help with hearing loss wouldn’t offer any benefits for cognitive decline.
Studies that have investigated the Common Cause Theory have generally found that their findings are consistent with the theory insofar as cognitive decline does not seem to cause hearing loss or vice versa. Unfortunately, no studies have yet identified what this common cause might be.
The Cascade Theory argues that hearing loss is just the beginning of a series of negative events that ultimately leads to cognitive decline. This theory is a kind of use-it-or-lose-it theory which necessitates a causal relationship between hearing loss and cognitive decline. Essentially, this theory posits that sensory deficits (like those seen in hearing loss) reduce neural inputs to the brain, which eventually leads to neural atrophy (the death of nerves) and cognitive decline.
There are studies that suggest the Cascade Theory may offer a good explanation of how people can simultaneously develop hearing loss and cognitive decline as they age. That being said, the Cascade Theory cannot eliminate the possibility that some common factor causes neural atrophy and, ultimately, sensory and cognitive decline.
Overdiagnosis or Harbinger Theory
This aptly-named theory argues that overdiagnosis is the reason why we see an association between hearing loss and cognitive decline. Proponents of the theory argue that since many cognitive tests are given verbally, people with hearing loss may not fully comprehend the tasks they are given and are, as a result, labeled as having low cognitive function. Additionally, someone with cognitive impairment may not fully understand the instructions they are given for a hearing test, which could result in inaccurate results.
Some studies have found that people with hearing loss are more likely to receive lower test scores on cognition tests and vice versa, but while it is possible that some people have been misdiagnosed, it is fairly unlikely that all cases of simultaneous hearing loss and cognitive decline are due to misdiagnosis.
Ultimately, while there are a number of different theories out there that purport to explain the connection between hearing loss and cognitive decline, the most widely accepted is the Common Cause Theory. However no one particular common cause has been identified in the literature, so more research is needed.