The Sound of Music
Making Music Enjoyable for Cochlear Implant Users
From a 35,000-year-old bird-bone flute to today’s Tik Tok hits, music is a diverse, expressive and persistent kind of sound. It can convey emotion without words and is found in every culture. But what happens when music becomes garbled?
Cochlear implants (CI) are good at helping those with hearing loss perceive speech, but the devices are not so good at conveying the complexity of pitches found in music, said Dr. Charles J. Limb of the University of California, San Francisco. In his recent Clinical Center Grand Rounds lecture “Music for Deaf Ears: Cochlear Implant-Mediated Perception of Music,” he explored why CI users often have trouble enjoying music and how researchers might address it.
“Hearing is fundamentally an electric process,” Limb said. The temporal bones, which form the sides and base of the skull and surround the ear canal, translate vibrational energy (that is, sound) into electrical signals in the brain.
Sound is transmitted from the ear canal to the eardrum and then through the bones of the middle ear, which amplifies the vibrations. The vibrations travel into the fluid of the inner ear (in the snail-shaped cochlea) and trigger neural impulses in the auditory nerve. The auditory nerve sends those neural impulses to the brain, where they are translated into sound.
As a self-described “music junkie,” Limb was fascinated by our ability to perceive music. Sinusoids, or pure tones, are “blips and beeps” that he studied in his early days of otology and neurotology. Sinusoids are a far cry from music, but they are actually “a very difficult feat for the auditory system,” Limb said. “And we know [this] because when we try to recreate that experience in a CI user, we basically fail.”
He pointed to a survey of CI users that evaluated their music taste before hearing loss and after CI implantation. Participants had varying levels of interest in music before hearing loss, but, after CI implantation, almost 40 percent indicated that they no longer enjoyed listening to music.
What Goes Wrong?
Cochlear implants bypass damaged portions of the inner ear and stimulate the auditory nerve. Inner ear damage may be congenital or may be caused by aging or external factors such as noise exposure or a viral infection.
In a CI, electrodes do the work of transmitting sound signals to the auditory nerve in the form of electricity. However, the roughly 20 electrodes that are part of the CI cannot convey the complete range of sound that a fully functional auditory system can.
When graphed, music covers a much wider range of volume (decibels) and pitch (frequency or Hertz) than language.
“If you look at the acoustic properties of music as opposed to language, you find right away that music is a more difficult stimulus than language,” Limb explained. “On basic sound dynamics and frequency range, [music is] just a more expansive sound.”
Not only is music more expansive, but it is also more complex because it often involves multiple instruments. “And because it is a complex sound…you can then deconstruct it and use it as a way to understand the limits of one’s hearing,” Limb reasoned.
All About That Bass
What does music sound like to a CI user? Out of five musical elements evaluated (pitch, rhythm, timbre, sound quality and dynamics), only rhythm was still clear to CI users. Music tends to sound out of tune, the dynamic range (how loud or soft the music is) is limited and the general sound quality is poor. This degradation of sound can be especially difficult for individuals who lost hearing later in life and are no longer able to hear music the way they used to.
Pitch and sound quality seem to be the biggest obstacles for CI users, Limb has found. He has identified three potential pathways for improving music: change the CI, change the brain or change the music itself.
How would these work? You could change the way the electrodes fire by making them more precise.
“Right now, it’s like trying to water a single blade of grass with a garden hose,” Limb explained. If you could narrow the current of electricity, the CI user could hear pitches more precisely.
Another CI modification involves use of a “phantom electrode,” which involves guiding electricity into the apex of the cochlea (the innermost part of the spiral) with the hope of hitting lower-frequency neurons. The effect is what Limb calls a “bass boost,” which helps CI users hear lower-pitched sounds and can actually improve the sound quality of both music and spoken language.
Interestingly, CIs can benefit people who still have some hearing left. A person who can still hear lower frequencies can benefit from “dual-modality hearing.” The CI restores their ability to hear higher frequencies and the individual’s natural ability to hear at lower frequencies enriches the effects of the CI. These folks also have better pitch perception.
This effect is also observed in hearing individuals; Limb used the example of Yeah by Usher, played both with and without the bass line. The song sounded “better” with bass.
Future CIs may also be able to be “tuned” or adjusted more precisely to the placement of the electrodes in the cochlea and the user’s specific needs.
What about changing the brain? It’s not so much changing as training, Limb explained. People with “perfect pitch,” who can recognize the pitch of any note or produce any given note without a musical guide, do not necessarily have better hearing than the rest of the general population; they just have better training.
“The difference is in [the] brain’s ability to interpret complex combinations of pitches,” which is accomplished through training, said Limb. Many people with perfect pitch were exposed to music at a very young age. CI users also go through auditory training, but their version is speech-based.
Adding music training might improve CI users’ ability to detect different frequencies. In fact, young children who have received CIs and then undergo music education classes are better at distinguishing between pitches than CI users with no musical training.
Tonal languages such as Taiwanese, in which changing the inflection of your voice changes the meaning of a word, are also beneficial for teaching pitch perception to CI users. Children with CIs in Taiwan were markedly better at distinguishing between pitches than their English-speaking counterparts.
“It’s not because their CIs were better,” Limb said, “but…because they’re living in a tonal language-based society, where they must give meaning to these tone sweeps and they’re better able to do it.” And, the earlier a child is taught to do this, the more successful they will be.
Reasons for Hope
Bass boosts, music education, improving CI placement—there are multiple opportunities for improving the sound of music for CI users.
Music has enriched humans’ lives for thousands of years. Hopefully, the work of Limb and other researchers can restore this experience to CI users.
“If you can hear music better, you can hear everything better,” said Limb.
View the archived lecture at https://videocast.nih.gov/watch=45354.