NE Dallas News

For Cochlear Implant Users, Cause of Halting Speech May Be Clearer

Children who receive cochlear implants before fully developing speech and language skills generally attain those skills more slowly than their typically hearing peers and eventually develop a slower rate of speaking. The reason for those delays has been unclear.

To understand what factors might be responsible, University of Texas at Dallas researchers studied the language processing of teenagers who received implants before age 5. The results of the study, published Dec. 12 in the Journal of Speech, Language, and Hearing Research, suggest that the bottleneck occurs at the level of syntax processing and production, not with merely interpreting sounds.

The cochlear implant has been a fabulous technology, but some people struggle with halting speech,” said Dr. William Katz, professor of speech, language, and hearing in the School of Behavioral and Brain Sciences. “Cochlear implant recipients have slowness across the board, though it is often not noticeable. Researchers have not known what to make of this.”

Cochlear implants, intended for people for whom hearing aids are insufficient, provide sound using electrical stimulation of the auditory nerve. Children who receive cochlear implants at a young age exhibit varying degrees of delayed articulation, difficulty with phonological processing and reduced vocabulary.

“Cochlear implants cypher a signal to the auditory nerve. Recipients don’t hear what we hear; their brains map in that signal,” said Katz, who also directs the Speech Production Lab at UT Dallas’ Callier Center for Communication Disorders.

The researchers measured speaking rates for 10 prelingually deaf teenagers who received cochlear implants before the age of 5 and 10 age-matched typically hearing teenagers.

“There has been little evidence to disentangle the causes and effects behind the speech difficulties suffered by cochlear implant users,” Katz said. “To address this, we chose three sets of stimuli to represent different levels of processing stages of language and grammar, hoping to determine which level contributes most to speech slowdown, and how memory plays a role.”

They first used strings of non-word, consonant-vowel syllables; then real words; then complex sentences. They also examined the relationship between these speaking rates and immediate verbal memory, measured by digit span forward and backward tests — repeating a list of digits, then reversing them.

“It’s combining words that creates the processing logjam. The cochlear implant recipients seem to have a compensatory need for extra verbal memory at the level of syntax. And that’s intriguing.”Dr. William Katz, professor of speech, language, and hearing in the School of Behavioral and Brain Sciences

“We found a significant difference for prelingually deaf teenagers using cochlear implants compared with typically hearing counterparts — but only for the sentence repetition task, not for non-word string or word string repetition tasks,” said Sujin Shin PhD’18, lead author of the paper and now an assistant professor of communication sciences and disorders at the University of Redlands in California. “These results indicate that those difficulties were related to syntactic processing — the combination of words in language to convey meaning as a whole.”

Previous research has suggested that phonological loop theory might explain the relationship between immediate verbal memory and speaking rate, theorizing that verbal information is stored in a time-limited loop, causing slower speakers to memorize less.

“Our results contradict this idea that one memory component holds auditory information for a short time, cycling it in your mind briefly,” Katz said. “That theory would predict an across-the-board problem for cochlear implant users — they would have had equal difficulty with all types of auditory stimuli.”

Instead, the study supports the idea that more general issues with controlled processing have a greater influence on speaking rate.

“It’s not at the level of processing syllables or non-words; it’s combining words that creates the processing logjam,” Katz said. “The cochlear implant recipients seem to have a compensatory need for extra verbal memory at the level of syntax. And that’s intriguing.”

Katz said the findings align with anecdotes from cochlear implant users about tiring out. 

“They talk about being exhausted — being mentally fatigued and needing rest, going so far as to occasionally take the implants out,” he said. “The notion of extra processes being required strikes a chord with me, because it seems that more work is taking place in the brain.”

There are several theories regarding factors for slowed speech in cochlear implant users. Skills at establishing grammatical relations may be underdeveloped due to prolonged degraded speech input in early life. There could be a neural adaptation component, where other systems of the early developing brain have taken over auditory regions due to the lack of input.

“When that input is suddenly added, the brain might have to work harder to deal with this other kind of signal that the implant provides,” Katz said.

There also may be a relationship between the speech slowdown and so-called clear speech — the slower, more deliberate speech that those with hearing difficulties often receive.

“If children receive frequent instruction to speak clearly and slowly, and often hear this pattern of speech from others who are aware of their issue, it may generally slow their speech, while also increasing pitch range and voice intensity,” Katz said. “Paradoxically, this may mean that training using more informal, natural speech in children with cochlear implants is important.”

These findings stress that syntactic processing should be emphasized in treatment of prelingual cochlear implant recipients, and that working memory therapy may also be useful.

“I know from when I first came to the U.S. and started to speak English that it is frustrating to be unable to communicate as quickly as you want. Some cochlear implant patients express similar frustrations that their speech is not fluent, smooth or at their desired speed,” Shin said. “One of the take-home messages here is that it’s not just a speech issue; it could be related to complex language processing and executive functions. More research is needed because it is complicated to weigh these factors.”

Other study authors are associate professor Dr. Andrea Warner-Czyz from the Department of Speech, Language, and Hearing and retired research scientist Dr. Ann Geers. The work was supported by the American Speech-Language-Hearing Foundation and UTD’s PhD Research Small Grants program.

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