Research Roundup updates HR readers on some of the latest research and clinical findings related to hearing health care. Where appropriate, sources and original citations are provided, and readers are encouraged to refer to the primary literature for more detailed information. Additionally, related articles can be found and keywords can be searched in the HR Online Archives.
New Hearing Mechanism Discovered
Recent research at the Massachusetts Institute of Technology (MIT) found that the tectorial membrane selectively picks up and transmits energy to different parts of the cochlea via a kind of wave that is different from that commonly associated with hearing. The findings fundamentally change scientific understanding of the inner ear and could help lead to processes that will better restore human hearing.
The research described in the advance online issue of the Proceedings of the National Academy of Sciences was the product of MIT professor Dennis M. Freeman, MIT graduate student Roozbeh Ghaffari, and research scientist Alexander J. Aranyosi.
It has been known for more than 50 years that inside the cochlea sound waves are translated into up-and-down waves that travel along the basilar membrane. But the MIT research team has now found that a different kind of wave, a traveling wave that moves from side to side, can also carry sound energy. This wave moves along the tectorial membrane, which is situated directly above the sensory hair cells that transmit sounds to the brain. This second wave mechanism is poised to play a crucial role in delivering sound signals to these hair cells.
In short, the ear can mechanically translate sounds into two different kinds of wave motion at once. These waves can interact to excite the hair cells and enhance their sensitivity, “which may help explain how we hear sounds as quiet as whispers,” Aranyosi says. The interactions between these two wave mechanisms may play a key part of how we are able to hear with such fidelity—for example, knowing when a single instrument in an orchestra is out of tune.
“We know the ear is enormously sensitive” in its ability to discriminate between different kinds of sound, Freeman says. “We don’t know the mechanism that lets it do that.” The new work has revealed “a whole new mechanism that nobody had thought of. It’s really a very different way of looking at things.”
The team’s discovery may have a direct impact on how cochlear mechanisms are modeled. “In the long run, this could affect the design of hearing aids and cochlear implants,” Ghaffari says.
Because the tectorial membrane is so tiny and so fragile, people “tend to think of it as something that’s wimpy and not important,” Freeman says. “Well, it’s not wimpy at all.” The new discovery “that it can transport energy throughout the cochlea is very significant, and it’s not something that’s intuitive.”
The research was funded by the National Institutes of Health. Source: MIT
Ghaffari R, Aranyosi AJ, Freeman DM. Longitudinally propagating traveling waves of the mammalian tectorial membrane. PNAS online. Available at: www.pnas.org/cgi/content/abstract/0703665104v1. Accessed October 30, 2007.
Planum Temporale Identified as Location for “Sound Space”
While the visual regions of the brain have been intensively mapped, many important regions for auditory processing remain “uncharted territory.” Now, researchers at the Hebrew University of Jerusalem and elsewhere have identified a region responsible for a key auditory process: perceiving “sound space,” the location of sounds, even when the listener is not concentrating on those sounds.
The findings settle a controversy in earlier studies that failed to establish the auditory region, called the planum temporale, as responsible for perception of auditory space by default.
The researchers, led by Leon Y. Deouell, PhD, of the Psychology Department and the Interdisciplinary Center for Neural Computation of the Hebrew University, and colleagues from the University of California, Berkeley, and the Weizmann Institute of Science, published their findings in the September 20 issue of the journal Neuron.
Studies by other researchers had shown that the planum temporale was activated when people were asked to perform tasks in which they located sounds in space. However, many researchers believed that the region was responsible only for intentional processing of such information. And, in fact, previous studies had failed to establish that the planum temporale was responsible for automatic, nonintentional representation of spatial location.
Previous research done by Deouell and others has shown that some patients with brain damage may be specifically impaired in this function. Understanding how the normal brain machinery for this function is organized may help to understand why it breaks down and, eventually, how to mend it.
In their work, the researchers used an improved experimental design that enabled them to more sensitively determine the brain’s auditory spatial location center. For example, they presented their human subjects with sounds against a background of silence, used headphones that more accurately reproduced sound location, and used noise with a rich spectrum, which has been shown to be more readily locatable in space. They also used sounds recorded from microphones placed in each subject’s own ears, and then played the same sounds back, thus tailoring the sounds specifically to the subjects’ own heads and ears.
In their experiments, the research team presented bursts of the noise to the volunteers wearing the headphones while the subjects’ brains were scanned by functional magnetic resonance imaging. In this widely used brain-scanning technique, harmless magnetic fields and radio waves are used to image blood flow in brain regions, which reflects brain activity in those locations. The subjects were instructed to ignore the sounds. And, to divert their attention, they either watched a movie with the sound turned off or were given a simple button-pushing task.
When the position of the noise bursts was varied in space, the researchers found that the planum temporale in the subjects’ brain was, indeed, activated. Additionally, the greater the number of distinct sound locations subjects heard during test runs, the greater the activity in the planum temporale.
The researchers thus concluded that their experiments “suggest that neurons in this region represent, in a nonintentional or preattentive fashion, the location of sound sources in the environment.”
Working with Deouell on the project were Aaron S. Heller of the University of California, Berkeley; Rafael Malach of the Weizmann Institute of Science; and Mark D’Esposito and Robert T. Knight of the University of California, Berkeley. Source: American Association for the Advancement of Science.
Deouell LY, Heller AS, Malach R, D’Esposito M, Knight RT. Cerebral responses to change in spatial location of unattended sounds. Neuron. 2007;55:985-996.
Cartilage Shield for Some Chronic OM Infections
Inserting a “shield” of cartilage into the inner ear is a less invasive and more cost-effective alternative to membrane reconstruction when treating hearing loss in selected patients suffering from chronic middle ear infections (otitis media), according to a new study published in the June 2007 edition of Otolaryngology–Head and Neck Surgery.
The study’s authors determined that, by inserting a Type III cartilage shield through tympanoplasty as a way to replace damaged tympanic membranes, patients with hearing loss of this kind will experience, on average, an 11.22 dB improvement in hearing quality. The study monitored 52 patients treated with a cartilage shield insertion over a 7-year period.
According to the study’s authors, the method achieved results similar to reconstructing the tympanic membrane through alloplastic partial ossicular prostheses (PORPs); however, inserting PORPs is considered more invasive and costly, and in some cases is not a viable option.
The study’s authors are Efthymios Kyrodimos, MD; Aristides Sismanis, MD; and Daniel V. Santos, MD, all of whom are associated with the Virginia Commonwealth University Medical Center in Richmond, Va.
Kyrodimos E, Santos D, Sismanis A. AMTYPE III cartilage shield tympanoplasty: Hearing results. Otolaryngol–Head Neck Surg. 2007;135(2):P52.
New Cell Culturing Method Pumps Up the Volume
In a breakthrough that will likely accelerate research aimed at cures for hearing loss, tinnitus, and balance problems, scientists have perfected a laboratory culturing technique that provides a reliable new source of cells critical to understanding certain inner-ear disorders.
Damage to hair cells is a key factor in hearing and balance loss, and while birds, fishes, and amphibians can quickly regrow damaged hair cells, humans cannot. Until now, scientists seeking clues to this problem have been hampered by difficult procedures required to gather these cells for their research.
In the September 24-28 early edition of the Proceedings of the National Academy of Sciences (PNAS), MBL Whitman Investigators Zhengqing Hu and Jeffrey Corwin, both of the University of Virginia School of Medicine, describe a new technique for isolating cells from the inner ears of chicken embryos and growing them in a laboratory. The scientists achieved these results by inducing avian cells to differentiate into hair cells via a process known as mesenchymal-to-epithelial transition.
Hu and Corwin were able to freeze and thaw the cultured cells, then grow new cells from the thawed cultures—a discovery that will make hair cells accessible to more researchers.
The study of hair cells is crucial to understanding hearing loss because hair cells are a precious commodity in humans. We are born with a limited number of these sound detectors in each ear, which can be easily damaged by age, certain illnesses, loud noises, and adverse reactions to medications. Once damaged, the cells do not grow back, causing hearing and balance problems.
“Until now, scientists working to understand many inner ear disorders had to resort to difficult microdissections to gather even small numbers of these cells, which limited the types of research that could be pursued and slowed the pace of discoveries,” Corwin says.
The availability of vials of frozen cells that can be induced to form hair cells should remove a significant barrier to progress toward the development of treatments for the more than 20 million Americans who have hearing loss and balance problems. Source: American Association for the Advancement of Science.
Hu Z, Corwin JT. Inner ear hair cells produced in vitro by a mesenchymal-to-epithelial transition. PNAS. 2007; 104: 16675-16680.