WEDNESDAY, March 6, 2002 (HealthDayNews) -- There's more to sound than meets the ear -- or more specifically, the auditory nerve.
The ear splits sounds into two complex layers, and new research suggests that while one layer is crucial for understanding speech, the other contains information critical for pitch perception and localizing sounds in space.
Scientists say these findings could point to ways to design cochlear implants that deliver a richer range of auditory information to people whose deafness results from neural disruption of the hearing system.
The study appears in tomorrow's issue of the journal Nature.
People who require cochlear implants have sensory-neural hearing loss. The cells that produce hairs that translate vibrations from sounds into electrical signals, and then pass them on to the auditory nerve, are dead. Roughly 21,000 Americans, half of them children, have a cochlear implant, which transmits a limited amount of sound information directly to the auditory nerve.
Dr. Bertrand Delgutte, who heads the laboratory in Speech and Hearing Bioscience Technology where the research was done, came up with the idea of splitting sounds into their fast-varying and slow-varying parts, also known as "fine structure" and "envelope," recombining them to make new sounds.
Delgutte suspected the recombined sounds could be useful for studying several aspects of sound perception.
Lead author Zachary M. Smith, who works in Delgutte's laboratory at the Harvard-MIT Division of Health Sciences and Technology and has an interest in cochlear implants, tested the importance of envelope and fine structure in auditory perception.
"Our results give us some insight into what may be important for cochlear implant users," says Smith.
The researchers looked at speech and pitch perception, as well as sound vocalization, meaning how the location of a sound is determined, in six people who speak American English as their native language.
"We found that for speech perception, it's the envelope that's most important, and for music and sound localization, it's the fine structure that's most important," Smith says.
"One of the reasons why this is so interesting is that many cochlear implants only use envelope information," Smith says. "This makes sense, since cochlear implants can only deliver a limited amount of information and they are optimized for speech. We suggest that if there were a way to deliver more fine structure information via cochlear implants, then pitch perception and sound localization would be improved."
"When we speak, the way that we inflect the pitch of our voice adds meaning," he says. Smith adds that in tonal languages, such as Chinese, pitch is crucial for determining the meaning of single words.
The study "highlights that there are two different aspects to speech," says Robert V. Shannon, the head of auditory implants and perception at the House Ear Institute in Los Angeles. "There's understanding the meaning of the words, which is almost independent of the quality of the speech that you hear."
"People with cochlear implants today hear and understand words very well," Shannon says. "Most of them can speak on the telephone almost normally. But the quality of the sound that they hear is really not very good." Shannon compares it to playing the piano with a fist rather than fingers. "Cochlear implant patients can understand speech very well, but they have a very hard time with music because they're not getting the fine structure information."
According to Smith, delivering fine structure information through cochlear implants would be helpful, but it would be a technical challenge.
Smith says one possibility could involve changing cochlear implant speech processors, which convert sounds to electrical signals that are sent to the implant in the ear, to include fine structure information. "But this remains to be tested," he says.
While Shannon cautions that it may simply be too complex to deliver fine structure information via cochlear implants, he says studies like this are necessary to find out whether the step to the next generation of implants is possible.
"We're starting to understand what cues are important for intelligibility, and what cues are important for quality and music melody," Shannon says. "Understanding that will help us try to design devices that can provide both cues."
What To Do: Find out about cochlear implants from the National Institute on Deafness and Other Communication Disorders, the Cochlear Implant Association Inc. or the Alexander Graham Bell Association for the Deaf and Hard of Hearing.
