Understanding the Relationship

During a presentation held at the 169th Meeting of the Acoustical Society of America, researchers from the University of Colorado suggested that the portion of the brain devoted to hearing can become reorganized—reassigned to other functions—even with early-stage hearing loss, which plays a role in cognitive decline.

According to an ASA announcement, Anu Sharma, PhD, a researcher in the Department of Speech Language and Hearing Science at the University of Colorado, applied fundamental principles of neuroplasticity to determine how the brain adapts to hearing loss, as well as the consequences of those changes.

Since this initial study by Dr. Sharma and her team, other tracking studies have also found supporting data. “In a study that tracked 639 adults for nearly 12 years, Johns Hopkins expert Frank Lin, M.D., Ph.D, and his colleagues found that mild hearing loss doubled dementia risk. Moderate loss tripled risk, and people with severe hearing impairment were five times more likely to develop dementia.”

Sharma and her colleagues in the Brain and Behavior Laboratory used electroencephalographic (EEG) recordings of adults and children with deafness and lesser hearing loss to gain insights into the ways their brains respond differently from those of people with normal hearing.

hearing loss and cognitive decline

Adults with early-stage age-related hearing loss (right) show decreased activation of the hearing portion of the brain compared with normal hearing age-matched adults (left).

Sharma explained that EEG recordings involve placing multiple tiny sensors on the scalp, which allows researchers to measure brain activity in response to sound stimulation. For her study, sound simulation, such as recorded speech syllables, was delivered via speakers to elicit a response in the form of “brain waves” that originate in the auditory cortex—the most important center for processing speech and language—and other areas of the brain.

hearing loss and cognitive decline

Adults with mild age-related hearing loss (right) show brain reorganization in hearing portions of brain, which are recruited for processing visual patterns. This is not seen in age-matched adults with normal hearing (left).

“We can examine certain biomarkers of cortical functioning, which tell us how the hearing portion of a deaf person’s brain is functioning compared to a person with normal hearing,” Sharma said.

ASA reports that Sharma and other researchers have discovered that the areas of the brain responsible for processing vision or touch can recruit areas in which hearing is normally processed, but which receive little or no stimulation in deafness. This is called “cross-modal” cortical reorganization and reflects a fundamental property of the brain to compensate in response to its environment.

hearing loss and cognitive decline

A good cochlear implant user shows normal brain activations in response to visual and tactile stimulation. An average cochlear implant user shows recruitment of hearing brain centers for processing vision and tactile stimulation.

Studies conducted by Sharma and colleagues have also included cochlear implant patients.

“We find that this kind of compensatory adaptation may significantly decrease the brain’s available resources for processing sound and can affect a deaf patient’s ability to effectively perceive speech with their cochlear implants,” said Sharma.

Sharma’s group has demonstrated that charting brain functioning in patients with cochlear implants is a valuable tool to help predict their outcomes. “If a deaf child shows cross-modal reorganization—by vision, for example—it allows us to determine the optimal rehabilitation strategy for that particular child,” she said.

Sharma and her research team also reportedly made the discovery that cross-modal recruitment of the hearing portion of the brain by the senses of vision and touch happens not only in deaf patients, but is also clearly apparent in adult patients with only a mild degree of hearing loss.

“The hearing areas of the brain shrink in age-related hearing loss,” said Sharma. “Centers of the brain that are typically used for higher-level decision-making are then activated in just hearing sounds. These compensatory changes increase the overall load on the brains of aging adults. Compensatory brain reorganization secondary to hearing loss may also be a factor in explaining recent reports in the literature that show age-related hearing loss is significantly correlated with dementia.”

According to Sharma, this finding has important clinical implications for developing early screening programs for hearing loss in adults. The study results also suggest that age-related hearing loss must be taken seriously, even in its earliest stages.

“One in three adults over the age of 60 has age-related hearing loss,” Sharma said. “Given that even small degrees of hearing loss can cause secondary changes in the brain, hearing screenings for adults and intervention in the form of hearing aids should be considered much earlier to protect against reorganization of the brain.”

“Our goal is to develop user-friendly EEG technologies, to allow clinicians to easily ‘image’ the brains of individual patients with hearing loss to determine whether and to what degree their brains have become reorganized,” Sharma said. “In this way, the blueprint of brain reorganization can guide clinical intervention for patients with hearing loss.”

Dementia affects many individuals, relationships and families. If it can be stopped or slowed early by addressing hearing loss, you may want to schedule a visit with your local audiologist.

Source: Acoustical Society of America (ASA); University of Colorado
Images used with permission granted by Dr. Anu Sharma, University of Colorado, Boulder