“And over long periods of time, being exposed to loud noise,” said Feeney, “we think there could be an effect on more complicated signals. Maybe you hear tones okay, but you might have a deficit in hearing a more complex signal, requiring more of those eighth nerve fibers to respond – say, speech. A lot of this right now is theory. And work like the kind Naomi and Dawn are doing is on the forefront of trying to uncover what actually is going on with humans.”
In his own laboratory, Feeney focuses much of his work on tests that can help measure how the inner ear receives and processes more complicated auditory information. One study, which he’s doing in collaboration with Douglas Keefe, PhD, of the Boys Town National Research Hospital in Omaha and Lisa Hunter, PhD, of the Cincinnati Children’s Hospital Medical Center, is examining the ability of a more versatile test battery: wideband acoustic immittance (WAI), which evaluates the middle ear’s response over a broad range of frequencies and ear canal pressures.
Though nearly everyone with hearing loss can be helped with hearing aids, only about 1 in 5 people use them, and their reasons for not using them vary widely.
Feeney is also at work studying the potential of a wideband test of otoacoustic emissions (OAE), the sounds that travel back out of the ear canal after an auditory signal has been received. “We want to include that as part of a test battery for veterans,” said Feeney, “because the otoacoustic emission test tells how the outer hair cells are functioning, and if you have damage to the outer hair cells you typically have hearing loss. Unfortunately, most veterans, and most adults in general, never have that tested as part of a hearing test.”
Wideband OAE tests can also help evaluate patients for ototoxicity, allowing clinicians to evaluative middle ear and hair cell function together. Feeney and his colleagues are proposing a study to test them at higher frequencies than are traditionally evaluated – extended high frequencies, as they’re known.
“The goal here,” Feeney said, “is to try to have the most sensitive test of ototoxicity that we can, and one that doesn’t have the false positives, due to middle ear dysfunction, that we might encounter from time to time on certain patients.”
The Auditory Vestibular Research Enhancement Award Program (AVREAP)
NCRAR’s work is complemented by the work of the AVREAP, headquartered at the Mountain Home, Tennessee VAMC. The program, conducted through five distinct laboratories, aims to improve the quality of life for veterans with hearing and balance disorders.
The AVREAP’s director, Faith Akin, PhD, also leads the program’s Vestibular/Balance Research Program, which explores problems – and the potential for solutions – associated with the inner ear’s vestibular organs. For the last 15 years, she and her AVREAP colleague Owen Murnane, PhD, have been studying the question of what happens to the vestibular system when people are exposed to loud noises. “We’d seen patients in the VA who reported ongoing balance problems following blast exposures from as long ago as the Korean Conflict,” Akin said. “It just didn’t fit with what we knew about the vestibular system and how it recovers from damage, and there were no human studies on the effect of noise on the vestibular system.”
Human electrophysiology testing using an electrode cap, as shown, can help to measure how speech sounds are coded by the brain. NCRAR photo
Akin and Murnane helped drive the body of research that zeroed in on the otolith organs, the gravity sensors of the inner ear. In fish and amphibians, otoliths are the organs of hearing, but it wasn’t proven until the 1990s that otoliths in mammals also responded to sound. During that same time period, investigators in Australia developed a method to record a sound-evoked otolith response from the neck muscles of humans. Akin and Murnane’s work refined this response as a functional evaluation of otolith organs in humans, and, in later studies, discovered a correlation between the sound-evoked otolith response and noise-induced hearing loss. They were among the first investigators to demonstrate a link between noise exposure and damage to the inner-ear balance system. More recently, they’ve extended this work to patients with dizziness following mild traumatic brain injury (TBI) and have observed a similar link.