With two ears and a cochlear implant, each ear is tuned differently.
David Landsberger – David.Landsberger@nyumc.org
New York University School of Medicine
Department of Otolaryngology – EAR-Lab
462 First Ave STE NBV 5E5
New York, NY 10016, USA
Popular version of paper 1aPP1, “Electrode length, placement, and frequency allocation distort place coding for bilateral, bimodal, and single-sided deafened cochlear implant users.”
Presented Monday morning, May 7th, 2018, 8:05-8:25 AM, Nicollet D2, 175th ASA Meeting, Minneapolis, Minnesota.
Imagine listening to the world with two ears that are tuned differently from each other. A key pressed on a piano would be perceived as different notes in the left and right ear. A person talking would sound like two different people simultaneously saying the same thing, one to each ear. This is in fact the experience for many people listening with two ears where one of the two ears has a cochlear implant.
The cochlea in a normal hearing ear is arranged “tonotopically.” That is, high frequencies are represented in the bottom (base) of the cochlea and low frequencies are represented at the top (apex) of the cochlea. The regions between the base and apex of the cochlea represent different frequencies and are ordered along the cochlea from low (in the apical region) to high (in the basal region) along the cochlea.
Cochlear implants take advantage of the tonotopic property using an array of electrodes inside the cochlea. Stimulation from an electrode placed deeper into the cochlea provides a lower pitch than an electrode placed closer to the base of the cochlea. Cochlear implant signal processing therefore provides information about low frequencies on apical electrodes and high frequencies on basal electrodes.
However, there is a mismatch between the frequency represented by a given electrode and the frequency expected by a normal ear at the same location. For example, the deepest electrode might represent 150-200 Hz but be placed in a location that expects approximately 1000 Hz. One factor effecting this relationship is the placement of the electrodes in the cochlea. This depends on electrode length, surgical placement, and size of the individual’s cochlea. Another factor is the “frequency allocation” which is the mapping of which frequency ranges are represented by each electrode . The result is that the world is presented pitch shifted (and warped) by a cochlear implant relative to what would be expected by a normal ear.
This distortion may or may not be an issue for traditional cochlear implant users who are bilaterally deaf and listen to the world via a single unilateral implant. For these users, although pitch may be transposed, the transposition is consistent and therefore may be easier to perceptually manage. However, it has become more common for cochlear implant users to listen to the world with two ears (i.e. a cochlear implant in each ear, or a cochlear implant in one ear with acoustic hearing in the other). In this situation, each ear will be differently transposed. This may result in a single auditory object being perceived as two independent auditory objects and may provide contralateral spectral interference. The bilateral listener with a cochlear implant will likely listen to the world with conflicting information provided to each ear.
In the following presentation, we will quantify the magnitudes of these distortions across ears. We will discuss limitations (and potential modifications) to electrode design frequency allocations to minimize this problem for cochlear implant users listening with two ears.
(Figure 1) audio files “chickenleg.wav” and “ring.wav”
“Two audio demonstartions of listening to sounds that are differently tuned in each ear. In each sample, a sound is presented normally to one ear and pitch shifted to the other ear. The first sample consists of speech while the second sample consists of music. These samples simulate only a pitch shift and not hearing loss or the sound quality of a cochlear implant. Note: demos should be played back over headphones.”
 D.M. Landsberger, M. Svrakic, J.T. Roland and M. Svirsky, “The Relationship Between Insertion Angles, Default Frequency Allocations, and Spiral Ganglion Place Pitch in Cochlear Implants,” Ear Hear, vol. 36, pp. e207-13., 2015.