It seems to be simply part of the natural process of growing old: as we age, our hearing becomes progressively worse, starting with high-frequency sounds. But compensating for the problem is more than a matter of turning up the volume. Most older adults, even those with good hearing, have trouble with certain aspects of auditory processing, such as picking out individual voices in a noisy restaurant. Clearly, age-related hearing loss involves more than just the ears, but researchers know very little about the neurological aspects of hearing loss. With the help of a new mouse model created by researchers at the University of Rochester Medical Center in New York State, though, that may soon change.
Robert Frisina, the lead researcher on the project, had already been using a mouse line known as CBA mice to study progressive hearing loss. Like humans, the CBA mouse gradually loses its ability to hear high-frequency sounds as it ages. The CBA mice don't breed very well, though, so Frisina decided to cross them with a more virile strain known as C57 Black 6. The C57 mice are known for their poor hearing but, much to Frisina's surprise, the resulting offspring not only bred better than the original CBA line, they also retained exceptional hearing into old age.
Frisina and his team had inadvertently created an animal model for studying the age-related hearing problems that occur in the brain rather than the ear. Some researchers suspect these problems stem from cognitive decline, but the complexity of the mammalian auditory system makes it difficult to determine cause and effect. "In most mammals, if you want to study age-related hearing loss in the brain, you have to go through an old ear, which distorts the signal," he says. "With this new mouse line, you don't have that problem. Suddenly you have a window into the aging brain that you didn't have before."
Frisina specializes in animal models of hearing loss, but he also works with humans at the nearby Rochester Institute of Technology, where his father, Robert Frisina, Sr., is the director of hearing and speech research. Frisina knew from his clinical research that about 5 percent of adults over 50 years of age have so-called "golden ears," meaning they can hear the full range of frequencies detectable by the human ear (roughly 20 hertz to 20 kilohertz). He thinks the mice from his new mouse line are the rodent equivalents of golden ear humans, and he has the experimental results to back him up.
Researchers test mouse hearing almost exactly the same way hospitals test hearing in newborn humans. Frisina and his team used a computer to play a series of tones in the mice's ears. Then they measured the number of hair cells vibrating inside the animals' cochleas. They also assessed the mice's hearing indirectly by taking EEG-like recordings while the tones were being played. In both tests, the new mouse line outperformed the CBA mice, although the difference was only significant during old age, which for mice is about two years old.
Frisina's group published their findings in the journal Neurobiology of Aging a few weeks ago, but the work on the golden ear mice has only just begun. First, Frisina would like to identify the genetic factors responsible for the golden ear phenomenon. Although the genetics behind deafness are fairly well defined, to date, no one has identified genes associated with age-related hearing loss, he says.
Meanwhile, Sandra Gordon-Salant, a professor of audiology at the University of Maryland, College Park, is more excited about the new mouse line's potential for anatomical and cognitive studies. "Now that there's this new animal model, researchers will be able to study speech processing and other cognition-related hearing deficits more easily," she says. Frisina notes that one of the reasons hearing aids are so problematic is that they don't account for the neurological aspects of hearing. A better understanding of how age-related changes in the brain stem and cortex contribute to hearing loss could lead to more effective hearing devices or even pharmaceutical treatments.
That would be welcome news to the more than one third of adults between the ages of 65 and 75 who suffer from hearing loss, according to the National Institute on Deafness and Other Communication Disorders. And if a recent report by researchers at Johns Hopkins University is any indication, that figure will likely grow over the years, as headphones and other noise sources cause hearing loss to strike at younger ages.