By Yishane Lee and Inyong Choi, Ph.D.
Support for a Theory Explaining Otoacoustic Emissions: Fangyi Chen, Ph.D.
It’s a remarkable feature of the ear that it not only hears sound but also generates it. These sounds, called otoacoustic emissions (OAEs), were discovered in 1978. Thanks in part to ERG research in outer hair cell motility, measuring OAEs has become a common, noninvasive hearing test, especially among infants too young to respond to sound prompts..
There are two theories about how the ear produces its own sound emanating from the interior of the cochlea out toward its base. The traditional one is the backward traveling wave theory, in which sound emissions travel slowly as a transverse wave along the basilar membrane, which divides the cochlea into two fluid-filled cavities. In a transverse wave, the wave particles move perpendicular to the wave direction. But this theory does not explain some anomalies, leading to a second hypothesis: The fast compression wave theory holds that the emissions travel as a longitudinal wave via lymph fluids around the basilar membrane. In a longitudinal wave, the wave particles travel in the same direction as the wave motion.
Figuring out how the emissions are created will promote greater accuracy of the OAE hearing test and a better understanding of cochlear mechanics. Fangyi Chen, Ph.D., a 2010 Emerging Research Grants (ERG) recipient, started investigating the issue at Oregon Health & Science University and is now at China’s Southern University of Science and Technology. His team’s paper, published in the journal Neural Plasticity in July 2018, for the first time experimentally validates the backward traveling wave theory.
Chen and his coauthors—including Allyn Hubbard, Ph.D., and Alfred Nuttall, Ph.D., who are each 1989–90 ERG recipients—directly measured the basilar membrane vibration in order to determine the wave propagation mechanism of the emissions. The team stimulated the membrane at a specific location, allowing for the vibration source that initiates the backward wave to be pinpointed. Then the resulting vibrations along the membrane were measured at multiple locations in vivo (in guinea pigs), showing a consistent lag as distance increased from the vibration source. The researchers also measured the waves at speeds in the order of tens of meters per second, much slower than would be the speed of a compression wave in water. The results were confirmed using a computer simulation. In addition to the wave propagation study, a mathematical model of the cochlea based on an acoustic electrical analogy was created and simulated. This was used to interpret why no peak frequency-to-place map was observed in the backward traveling wave, explaining some of the previous anomalies associated with this OAE theory.
Speech-in-Noise Understanding Relies on How Well You Combine Information Across Multiple Frequencies: Inyong Choi, Ph.D.
Understanding speech in noisy environments is a crucial ability for communications, although many individuals with or without hearing loss suffer from dysfunctions in that ability. Our study in Hearing Research, published in September 2018, finds that how well you combine information across multiple frequencies, tested by a pitch-fusion task in "hybrid" cochlear implant users who receive both low-frequency acoustic and high-frequency electric stimulation within the same ear, is a critical factor for good speech-in-noise understanding.
In the pitch-fusion task, subjects heard either a tone consisting of many frequencies in a simple mathematical relationship or a tone with more irregular spacing between frequencies. Subjects had to say whether the tone sounded "natural" or "unnatural" to them, given the fact that a tone consisting of frequencies in a simple mathematical relationship sounds much more natural to us. My team and I are now studying how we can improve the sensitivity to this "naturalness" in listeners with hearing loss, expecting to provide individualized therapeutic options to address the difficulties in speech-in-noise understanding.
2017 ERG recipient Inyong Choi, Ph.D., is an assistant professor in the department of communication sciences and disorders at the University of Iowa in Iowa City.
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