What are some of the differences between prelingually and postlingually deaf CI users? Previous research has shown that the auditory nerve degenerates slowly following deafness. The degree of degeneration is proportional to the duration of deafness: the longer the deafness period, the greater the auditory nerve loss. The Food and Drug Administration in 2000 approved CI implantation in children 12 months and older with severe to profound hearing loss, meaning that a 12-month-old infants who receive CIs have been without sound since birth.
The period of deafness, however, for postlingually deaf CI users can be anywhere from zero to 10 years or more, depending on patients' etiology. This means that the duration of deafness for pediatric CI users may be significantly shorter because they receive CIs when they are young. This is important because it suggests that prelingually deaf children may have greater neural survival than postlingually deaf adults. Prelingually deaf children with CIs might benefit more from CIs than postlingually deaf adults if speech understanding with a CI depends on the degree of nerve survival.
Acoustic vs. Electric Hearing
The story is not so simple, however, because another important factor influences CI outcomes. Postlingually deaf adults with CIs have been able to hear sound acoustically for a period of time. Depending on a patient's etiology and the onset of hearing loss, it could be 10 years or longer in cases of gradual, progressive hearing loss. These people have fully developed their auditory and spoken-language skills with their ears and brains during this time. This is not the case for prelingually deaf children who have had limited access to spoken language prior to receiving a CI. We need to be aware of a potentially huge difference between development of the auditory system with acoustic hearing via an intact ear and electric hearing via a CI.
We need to remember when evaluating auditory performance in children that a normal auditory system does not fully develop until about age 11, sometimes older. Prelingually deaf children with CIs are still developing their basic auditory function with electric hearing, suggesting that the auditory development pattern in this group of children may proceed quite differently from the development pattern seen in normal hearing children.
Music and Speech Perception
We recently studied performance and music and speech perception in prelingually deaf children with CIs using adult CI tests from our research program at the Virginia Merrill Bloedel Hearing Research Center at the University of Washington in Seattle. (Audiol Neurootol 2012;17:189.) We used five different tests: a monosyllabic word recognition test, or consonant-nucleus-consonant (CNC) word test; spondee word recognition in noise; a music perception test; spectral-ripple discrimination; and Schroeder-phase discrimination.
The CNC word test is used in almost all adult CI centers across the country. The spondee word recognition in noise test is designed to evaluate CI users' speech perception abilities without rhythmic or syllabic cues in the presence of background noise. The test includes 12 spondees, such as the words birthday and stairway. We also employed the University of Washington Clinical Assessment of Music Perception (UW-CAMP) test, which has been widely used in many CI programs in the United States and Canada since 2007. This test evaluates three basic music perception abilities: pitch discrimination, melody recognition, and musical instrument identification. It is important to understand how prelingually deaf children using CIs perform on the speech-in-noise and music perception tests because these are the most common difficulties that typical CI users face.
The spectral-ripple and Schroeder-phase tests do not use speech or music signals but artificially created sounds carefully designed to evaluate fundamental aspects of hearing abilities. The spectral-ripple discrimination test, for example, is useful to evaluate CI users' ability to distinguish two different spectral patterns. This skill is important for perceiving differences between certain speech sounds, especially vowels. The spectral-ripple discrimination performance by CI users has been shown to correlate with various types of speech perception abilities, including vowel perception, word and sentence recognition in quiet, and word recognition in babble or steady noise. The Schroeder-phase discrimination is designed to evaluate how well CI users can discriminate a rapid change in time and frequency domains. The stimuli used in this test sound like birds chirping.
We compared two groups of participants: prelingually deaf children with CIs and postlingually deaf adults with CIs. We tested 11 prelingually deaf children, ranging in age from 8 to 16 (mean age=12.1). The participant recruitment criteria included implantation before age 5 (mean age=2.4) with more than five years of CI use (range=5–16). All participants were native English speakers, bilaterally deaf, and had no residual hearing in either ear. They were all normally developing children except for their hearing loss.
No differences were found between child and adult groups for spectral-ripple discrimination, CNC word recognition, and spondee word recognition in noise. This result confirmed early findings that prelingually implanted children and postlingually implanted adults show similar speech perception performance. Adult-like spectral-ripple discrimination performance in CI children suggested that, at the ages tested, children and adults have similar sensitivity to changes in the frequency spectrum. We speculated that adult-like spectral sensitivity might have greatly contributed to this speech outcome in prelingually deaf children.
Schroeder-phase discrimination and music perception performance, except pitch discrimination, was generally poorer in children than in adults. It is perhaps discouraging to know that prelingually deaf children did not show similar performance in music perception to adults, but we need to understand that music involves more dynamic acoustic cues.
Temporal cues, for instance, are critical for timbre perception. Timbre makes one particular musical sound different from another, such as sounds produced by different instruments, and this attribute cannot be explained in terms of pitch or loudness. That prelingually deaf children showed worse performance in Schroeder-phase discrimination might be related to their poorer performance in music perception. It is possible that the poorer Schroeder-phase discrimination scores seen in the children might be partly accounted for by incomplete maturation of their temporal processing ability.
These findings suggest that it is not entirely clear how prelingually deaf children with CIs use their spectral and temporal sensitivities for speech understanding or music perception, although speech perception scores of children and adults with CIs are equivalent. Adult-like spectral-ripple discrimination and immature Schroeder-phase discrimination performance might suggest that children with CIs may place greater weight on spectral cues than on temporal cues for complex listening tasks.
Compared with speech perception tasks, melody and timbre recognition tasks may require more fine temporal sensitivity, leading children to perform worse than adults. These data, however, do not have to be interpreted negatively; we might expect that prelingually deaf implanted children's performance of musical tasks may improve over time as their temporal sensitivity matures. It may be important to provide this population with a proper sound encoding strategy, clinical map, and training program that can maximize their development in temporal sensitivity, or take maximal advantage of their reliance on spectral cues.
We would like to thank Liz Anderson and David Horn for their assistance with this article. Work supported by the NIH grants R01-DC007525, P30-DC04661, and F31-DC009755, and an educational fellowship from Advanced Bionics Corporation.
© 2012 Lippincott Williams & Wilkins, Inc.
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