Feature
Relearning to Hear and Speak
March 5, 2007
IU psychologist David Pisoni studies speech and hearing in children with cochlear implants
Once a week, Indiana University psychologist and cognitive scientist David Pisoni commutes from Bloomington to Riley Hospital for Children in Indianapolis. There, for the past 15 years, Pisoni has studied children with cochlear implants — small, surgically implanted electronic devices that can provide a sense of hearing to people who are partially or profoundly deaf.
Observing how some children (and adults) with cochlear implants learn to hear and speak at a high level, while others struggle, has had a profound affect on Pisoni's career-long quest to understand the complex interaction of speech and hearing in the brain.
"Working with these kids you come to see that the mind and the body and the world are all connected," Pisoni said. "The most profound change in my thinking about speech and hearing has been to look at these problems from a much broader perspective than just cognitive science, neuroscience, or speech science."
David Pisoni, IU professor of psychology and cognitive science.
Founded on basic and applied research in all three disciplines, cochlear implants represent a major breakthrough in hearing technology. Unlike a hearing aid, which only amplifies sound, a cochlear implant replaces damaged parts of the ear that translate sound waves into signals that are channeled via the auditory nerve to the brain. Consequently, cochlear implants can not only improve impaired hearing but also enable people who are completely deaf to comprehend sound.
Around 100,000 people worldwide have received cochlear implants. For many recipients, the implants allow them to gain what Pisoni calls a "sense" of hearing akin to listening to a radio station with less than perfect reception. Human speech processed through a cochlear implant typically sounds slightly distorted, obscuring fine details, but the timing and rhythm of speech sounds remain intact. And although conversation in a crowded restaurant or equally noisy atmosphere may remain out of reach, under quieter circumstances cochlear implants make face to face and phone conversations feasible.
Pisoni and other speech and hearing researchers remain confounded, though, by the fact that the success of cochlear implants varies widely among recipients. Some recipients who were formerly deaf and, consequently, also functionally mute, adapt quickly to implants and learn to not only make sense of human speech but also to speak clearly. For reasons that scientists have yet to pin down, though, even when the surgery is a success and the implant functions properly, others have difficulty adapting and experience delays and sometimes complete failure in hearing and speech.
"It's a function of neural processing and what the brain is doing with the [auditory] signal," Pisoni said. "We're doing behavioral tests that look at how learning and memory happens in the brain and how that relates to speech and hearing, but the bottom line is that nobody really understands how that works in all its complexity."
Pisoni's work with children born without hearing who received cochlear implants around the ages of 4 or 5 has helped clarify the relationship between hearing and speech. Likening his research to the so called "forbidden experiment" wherein a scientist would artificially deprive a newborn of sensory stimuli and observe how, or whether, the infant learns to speak, Pisoni has learned that speech is dependant on hearing.
"A formerly deaf child that adapts well to a cochlear implant will learn to speak," he said. "But kids that don't learn to hear with an implant don't acquire speech either. If you're not exposed to spoken language, you don't learn how to speak."
The implication, Pisoni said, is that we're not born with the ability to comprehend and produce language. Both processes are largely learned, and in the absence of hearing, spoken communication does not spontaneously develop. The earlier a deaf child is able to process sound, the faster and better able he or she is able to learn to speak. Consequently, doctors now recommend that children born with partial and total hearing loss receive cochlear implants as early as 12 months.
Pisoni remains intent, meanwhile, on continuing to investigate the puzzling discrepancies in hearing and speech development among cochlear implant recipients. His ultimate goal, Pisoni said, is to compile data that will help cochlear implant manufacturers build devices more sensitive to the wide variation in how individual brains interpret sound.
"Fifteen years ago people thought, let's just do the surgery, put in a sensory substitute, and that's that," Pisoni said. "But what we've learned is that the brain, nervous system, and environment play crucial roles in how well a given implant works. It's not like getting a knee implant, where you don't have to relearn how to walk. When you get a cochlear implant, the brain has to learn how to make sense of what it hears."
