Estimates are that millions of people have asymmetric hearing loss, where there is more hearing loss in one ear than the other. This can pose serious challenges to understanding speech and identifying where sounds are coming from, even though people may partially adapt to asymmetric inputs to the brain, says University of Massachusetts Amherst hearing researcher Richard Freyman.
The specific underlying causes of these hearing difficulties are poorly understood because of a lack of basic and applied research, he adds. “We need more information to inform clinical decisions faced by real people.” Now Freyman and his colleague Karen Helfer in the communication disorders department at UMass Amherst’s School of Public Health and Health Sciences have received a five-year, $1.6 million grant from the National Institutes of Health to advance understanding of asymmetric hearing loss and help this large population.
“We know that people adapt to asymmetric hearing thresholds to some extent, and we’re interested in how they do this. We don’t want to offer treatment that would interfere with this natural adaptation, but instead to identify what is helpful. Right now, we probably don’t know enough to guide either professionals or patients towards the most appropriate treatment options.”
Many people who have severe unilateral hearing loss, good hearing in one ear and poor hearing in the other, communicate just fine in face-to-face conversations in quiet environments. Problems show up when conditions are not perfect, such as when there is reflected sound and background noise, the researchers note. But because they do not have difficulty in optimal conditions, these people are often given no hearing aids or other assistive listening devices, Freyman adds. This may not be the best option, especially for school-aged children, who are at a significantly increased risk of needing to repeat a grade.
Possible interventions range from partially restoring hearing with a hearing aid, to rerouting signals from the poorer ear to the better ear, to surgical cochlear implantation in the poorer ear. “Knowing which of these vastly different treatment options is best for any particular personrequires a strong foundation of knowledge that we currently lack, and this absence of information has serious consequences,” he says.
As a psycho-acoustician, Freyman studies the relationship between sound and our auditory perceptions. In particular, he is interested in how we pick out the sound we want to listen to in a complex background of interfering sounds, and how we correctly locate sources of sound.
“This ability to localize sound is an important aspect of our auditory system,” he adds. “In our everyday experience it keeps us safe, for example, out of the way of a passing bus, and it also gives us a feeling of connection to our surroundings.”
Sound localization is largely based on the tiny time differences of less than one millisecond that occur between the two ears when sound is off to one side. In fact, our brains are sensitive to time differences of as little as 10 one-millionths of a second, Freyman says.
Acoustic research shows that in most indoor environments, for someone listening to a speaker who is more than about three feet away, the sound energy reflected off walls or objects is actually greater than energy from the source. Fortunately, the human auditory system is able to compensate for this and sound localization is still very accurate, Freyman says. Our brains can pay more attention to the sound arriving earlier, from the source, a phenomenon known as the “precedence effect.”
Some of Freyman and Helfer’s experiments will take place in a special anechoic chamber on the UMass Amherst campus, a room where there are no reverberations or echoes where the researchers can study how the brain processes sound arriving at different times in each ear and precisely introduce controlled reflections by presenting delayed sounds from secondary loudspeakers.
The effect of hearing impairment or loss in one ear on the precedence effect is unknown, Freyman says. The researchers plan what they believe will be the first studies on the many aspects of the precedence effect in listeners with asymmetric hearing loss. Results should lead to a better scientific understanding of how listeners with asymmetric hearing cope with their unbalanced hearing in realistic environments and to better-informed professional management of such hearing loss.
Freyman and Helfer’s grant from the National Institute on Deafness and Other Communication Disorders is a renewal of a sound localization project that began in 1992, likely making it one of the campus’ longest running research awards, he says.
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