Joshua Bernstein –

Kenneth Jensen –

Walter Reed National Military Medical Center
4954 N. Palmer Rd.
Bethesda, MD 20889

Jack Noble –
Vanderbilt University
2301 Vanderbilt Pl.
Nashville, TN 37235

Olga Stakhovskaya –
Matthew Goupell –
University of Maryland – College Park
7251 Preinkert Drive
College Park, MD 20742

Popular version of 1aPP2, “Measuring spectral asymmetry for cochlear-implant listeners with single-sided deafness”
Presented Monday morning, May 7, 2018
175th ASA Meeting, Minneapolis, MN

Having two ears provides tremendous benefits in our busy world: helping people to communicate in noisy environments, to tell where sounds are coming from, and to feel a general sense of three-dimensionality. People who go deaf in one ear (single-sided deafness) are therefore at a considerable disadvantage compared to people with access to sound in both ears.

Recently, cochlear implants have been explored as a way to restore some hearing to the deaf ear for people with single-sided deafness. A cochlear implant bypasses the normal inner-ear function, relaying sound information directly to the auditory nerve and brain via small electrical bursts.  While traditionally prescribed to people with two deaf ears, recent studies show that cochlear implants can restore some aspects of spatial hearing to people with single-sided deafness [1, 2].

The benefits that a cochlear implant provides to a person with single-sided deafness might not be as large as they could be because the device was never designed for this population. We know that for a given sound frequency, the cochlear implant stimulates the incorrect place in the cochlea (the snail-shaped hearing organ in the inner ear). Figure 1A shows the snail-shaped cochlea straightened into line. A normal-hearing ear processes the full frequency range (20-20,000 Hz) of from the one end of the cochlea to the other. However, cochlear implants deliver frequencies to the wrong cochlear locations because the device cannot be placed to allow access to the full range of frequency-specific nerve cells. Frequency mismatch could make it difficult for people with single-sided deafness to combine the sounds across the two ears, causing them to “hear double” instead of somewhat, although imperfectly, in stereo.

Figure 1.  A schematic of an unrolled cochlea showing how frequency mismatch arises because the cochlear implant electrode array (blue) cannot be inserted all the way to the end of the cochlea. (A) Programming a cochlear implant in a standard way leads to a frequency mismatch between the cochlear-implant (green) and normal-hearing ears (red). (B) Adjusting the cochlear implant frequency allocation could reduce or eliminate this mismatch. 

Legend: Sound examples, best experienced over headphones.

Simulation of hearing with a mismatched cochlear implant.

Simulation of hearing with a frequency-matched cochlear implant

Our research aims to reprogram cochlear implants to frequency-align the two ears for people with single-sided deafness (Figure 1B) by measuring where in the cochlea individual electrical contacts (electrodes) are stimulating. We compared three methods: computed-tomography (CT) scans (like an x-ray) to visualize electrode locations within the cochlea; having the listener compare the relative pitches of the sounds presented to the two ears; and having the listener judge small (~1ms) differences in the arrival time of sounds at the two ears [3]. The timing judgments – the only of the measurements that required listeners to use their two ears together – gave similar estimates of electrode location to the CT scans. In contrast, pitch measurements gave different estimates, suggesting that the brain rewired itself to accommodate pitch differences, but did not rewire itself for spatial hearing. Device programming based on either the timing or CT measurements shows the most promise to improve the ability to use the ears in concert with one another. Our next step will be to make these programming changes to see if they improve stereo hearing.

[The views expressed in this abstract are those of the authors and do not reflect the official policy of the Department of Army/Navy/Air Force, Department of Defense, or U.S. Government.]

[1] Bernstein, J., Schuchman, G., and Rivera, A, “Head shadow and binaural squelch for unilaterally deaf cochlear implantees,” Otology and Neurotology, vol. 38, pp. e195-e202, 2017.

[2] Vermeire, K., and Van de Heyning, P. “Binaural hearing after cochlear implantation in subjects with unilateral sensorineural deafness and tinnitus,” Audiology and Neurootology, vol. 14, pp. 163–171, 2009.

[3] Bernstein, J., Stakhovskaya, O., Schuchman, G., Jensen, K., and Goupell, M, “Interaural time-difference discrimination as a measure of place of simulation for cochlear-implant users with single-sided deafness,” Trends in Hearing, Vol. 19, p. 2331216515617143, 2018.

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