A Stanford neurobiologist teamed with child-development specialists at the University of Maryland to see if children with cochlear implants were able to meld their newly acquired hearing capability with their ability to read lips. In other words, did their brains process speech the same way as people who are born with the ability to hear?
In the Dec. 5 online edition of the Proceedings of the National Academy of Sciences, the researchers discuss how they used a simple auditory test and found that some children with cochlear implants fuse the visual and auditory aspects of speech - just like people with normal hearing. But the effect was only seen in children who received their implants before the age of 30 months, adding to the body of evidence suggesting that the earlier a hearing-impaired child receives a cochlear implant, the better.
"I see this as a fascinating experiment that reveals the tremendous capacity for plasticity in the developing brain," said Eric Knudsen, PhD, the paper's senior author and the Edward C. and Amy H. Sewall Professor at the Stanford University School of Medicine.
Most people don't appreciate that speech is a product of both hearing and vision, explained Knudsen, who is also chair of Stanford's Department of Neurobiology. The brain has learned that in conversation, the lips and face make certain gestures that always occur together with certain sounds. "The brain is always combining what it sees with what it hears and making the best guess at what was said," he said. In most situations, hearing speech alone is just fine - that's why we can talk on telephones. But in noisy situations, when hearing is poor, our brains rely heavily on lip and facial movements to figure out what someone is saying.
Knudsen has a long history of studying the abi lity of the brain to adapt to new circumstances and learn new skills. But most of his work is done with owls. The inspiration for the study in children came from a casual talk between Knudsen and Nathan Fox, PhD, director of the University of Maryland's Child Development Lab. In their conversation, they discussed a phenomenon called the McGurk effect, which is an illusion first reported in 1976. If a listener is presented with an audio recording of a single syllable while watching a synchronized video of a speaker's face mouthing a different syllable, in some cases a third syllable is heard, due to the brain's fusion of non-matching audio and visual information. It's an unnatural situation that a person wouldn't normally encounter, but it simply and powerfully reveals that speech perception is a product of both sound and sight.
"The McGurk effect is so amazing because it demonstrates clearly that speech is not just what you hear," said Knudsen. "If I thought I heard 'ba' but the lips look like they are mouthing 'ga' then it must have been 'da.'" Another classic example of the McGurk effect is that the sound "pa" combined with the lip movements for "ka" ends up being heard as "ta." It is impossible to hear the proper sound while viewing the speaker's lips, noted Knudsen.
Knudsen and Fox wondered if children born deaf would experience the McGurk effect after receiving cochlear implants. Funded by the National Institute on Deafness and Other Communication Disorders and the American Hearing Research Foundation, their team looked at a total of 36 children, ranging in age from 5 to 14, born profoundly deaf and each having had a minimum of one year with a cochlear implant. At the time of the study, they could perceive spoken language and communicate verbally. All of the children could read lips, but until they received hearing input from their implants they could not make the connection between lip movement and sound.
The only kids who fully merged the visual and auditory effects of speech were those who had received implants before they were 30 months old, which indicates that the earlier the implant is done, the better the chances for fully integrated speech perception in the brain. Kids who received implants later showed little evidence of the McGurk effect. Instead, they relied solely on what the lips were doing when they were presented with the conflicting auditory and visual information. As a result, they reported that the sound was the syllable being mouthed.
The paper's first author, Efrat Schorr, who was earning her PhD from the University of Maryland when the study was done, emphasized that their findings demonstrate just one of the influences that early cochlear implantation can have on children with hearing loss and it reinforces the importance of early detection and intervention for children with hearing loss. Children who need an implant would benefit from having it sooner rather than later, the authors conclude in their paper.
As a developmental psychologist, Fox said that he has always heard that earlier is better for interventions. "But there is not much empirical data in humans demonstrating the importance of early experience on brain development and behavior," he said. "This study shows just that."