Ann E. Geers
Department of Otorhinolaryngology, Head and Neck Surgery, University of Texas Southwestern Medical Center, and The Callier Advanced Hearing Research Center, University of Texas at Dallas School of Behavioral and Brain Sciences, Dallas, Tex., USA
Development of spoken language is an objective of virtually all English-based educational programs for children who are deaf or hard of hearing. The primary goal of pediatric cochlear implantation is to provide critical speech information to the child's auditory system and brain to maximize the chances of developing spoken language. Cochlear implants have the potential to accomplish for profoundly deaf children what the electronic hearing aid made possible for hard of hearing children more than 50 years ago. Though the cochlear implant does not allow for hearing of the same quality as that experienced by persons without a hearing loss, it nonetheless has revolutionized the experience of spoken language acquisition for deaf children. However, the variability in performance remains quite high, with limited explanation as to the reasons for good and poor outcomes. Evaluating the success of cochlear implantation requires careful consideration of intervening variables, the characteristics of which are changing with advances in technology and clinical practice. Improvement in speech coding strategies, implantation at younger ages and in children with greater preimplant residual hearing, and rehabilitation focused on speech and auditory skill development are leading to a larger proportion of children approaching spoken language levels of hearing age-mates.
Copyright © 2006 S. Karger AG, Basel
It is apparent that spoken language development occurs spontaneously in the presence of normal hearing from birth, given a typical linguistic and social environment, and is diminished by the early deprivation of auditory stimulation that occurs with prelingual hearing loss. Before hearing aids were widely available, deaf children missed many of the speech sounds that occurred in everyday life, and teaching them to use and understand spoken language relied largely on visual, kinesthetic and tactile cues. The advent of the wearable electronic hearing aid more than 50 years ago had a dramatic effect on the spoken language development of these children. Teachers soon realized that more normal speech development could be encouraged in deaf children by maximizing use of their limited residual hearing [1, 2]. This auditory approach, combined with an emphasis on early intervention, formed the basis for auditory-oral education of deaf children, as we know it today. New developments in hearing aids and ear molds permitted even severely hard of hearing children to detect most speech sounds, including the low intensity, high frequency sounds of speech, such as the /s / and /t/. When hearing aids were fitted early in life, and accompanied by appropriate intervention, many hard of hearing children were able to learn to understand and produce speech through everyday involvement in spoken language communication.
Electronic hearing aids were less successful in ensuring spoken language development in profoundly deaf children. Many years of intensive formal instruction by highly trained teachers was often required for deaf children to develop the ability to make use of other sensory modalities to understand and produce speech. Their language development proceeded at about half the rate of hearing children , and they demonstrated average language delays of 4-5 years by the time they entered high school . The reported speech intelligibility of children with profound deafness was quite variable, averaging as low as 19% in some studies  and as high as 76% in others .
The slow speech and language progress in profoundly deaf children achieved with oral methods promoted the development of various systems for providing visual support to spoken English. During the 1970s, different forms of signed English were widely used in classrooms across the country . The 'total communication' (TC) approach reduced the emphasis on audition and speech training compared to auditory-oral methods and increased visual language input, but the levels of English language competence exhibited by deaf children did not improve [8, 9]. Cued speech, a system developed to visually convey spoken language at the phonemic level using hand shapes and placements to complement and for decreasing ambiguities in lip reading, has been adapted to more than 40 languages. This system has been most widely used and evaluated in the French language  but has had limited application in educational programs in the US.
Another attempt to improve access to spoken English by profoundly deaf children was the advent of wearable multichannel vibrotactile aids in the 1980s. These aids were designed to present temporal and spectral aspects of speech through a body-worn vibrator. These devices were expected to be valuable complements to hearing aids and lip reading for comprehension of speech in the profoundly deaf . However, no improvement in speech or language skills over those achieved with hearing aids alone has been documented, even after several years of intensive training .
The first multichannel cochlear implant system was introduced in the US in 1984 and gained Food and Drug Administration (FDA) approval in 1990 for children aged 2 years and older. Since then, thousands of deaf children have been given access to the sounds of spoken language via a device that is quite different from anything previously available. Both hearing aids and cochlear implants are designed to provide discrimination of speech in hard of hearing and deaf children, but they do so in fundamentally different ways. A hearing aid amplifies speech and acoustically delivers it to the ear, while the cochlear implant converts speech into an electrical signal that is used to stimulate the auditory nerve directly through electrodes implanted in the cochlea. This new technology has the potential to accomplish for profoundly deaf children what the hearing aid made possible for hard of hearing children more than 50 years ago. Now, even children who obtain minimal benefit from amplified speech are able to access speech information electronically through a cochlear implant. The primary goal of pediatric cochlear implantation is to provide critical speech information to the child's auditory system and brain to promote the development of spoken language.
Assessing Outcomes of Cochlear Implantation in Children
The most well documented effect of cochlear implantation is a marked increase in children's ability to comprehend speech [13, 14]. In addition, the use of cochlear implants has provided significantly faster acquisition of speech production skills  and language development  than the use of hearing aids or vibrotactile aids. Though the cochlear implant does not allow for hearing of the same quality as that experienced by persons with normal hearing, it nonetheless has revolutionized the spoken language acquisition for deaf children. However, the variability in performance remains quite high, and the reasons for that are mostly unknown. Much of the recent research in pediatric cochlear implantation is concerned not only with documenting language achievements in this population, but also with determining what factors might influence the outcome. Documented reasons for poor performance include late age of implantation, poor nerve survival, inadequate fitting, insufficient cognitive skills, educational and social environments emphasizing manual communication and limited parental support . These factors are similar to those that contribute to poor performance in children who use hearing aids .
Most parents who choose a cochlear implant for their child report that the primary reason is to allow them to develop spoken language . The use of sensory aids such as cochlear implants and hearing aids is, however, only one of many factors that influence a deaf child's spoken language progress and that must be taken into account when assessing the benefits of cochlear implants. When evaluating the success of cochlear implantation with prelingually deaf children, the contributions from many intervening factors, several of which have undergone change since cochlear implants were first available, must be considered.
Identifying factors influencing speech and language outcomes in prelingually deaf children is complicated by the fact that both technology and clinical practices are evolving, so that research reports to date regarding the levels achieved by children with cochlear implants may already be obsolete. Furthermore, because prelingual profound hearing loss is a relatively rare occurrence, and because children receiving cochlear implants are even rarer, accumulating a sufficient number of participants to produce research findings that can be generalized to typical implant users is difficult. Large-sample studies typically assemble data collected over a number of years on patients who received implants over a wide time frame. This practice may lead to combining outcomes data from different generations of technology and intervention practices. The following sections describe factors that combine to make spoken language outcome a moving target for researchers seeking to identify the benefits of cochlear implantation for young children's development.
The technology of cochlear implants has been continuously improving since the Nucleus 22 device from Cochlear Corporation was introduced in 1984 [Loizou, this vol, pp 109-143]. The number of children achieving open set speech perception has increased with implementation of each new technology , and their ability to perceive speech in noise has also improved . Speech discrimination, speech production and language scores have been positively affected in children who receive updated speech processors  and electrode arrays [22, 23]. The full impact of cochlear implants on speech and language development may be evident only in children who have continuous use of the newest generations of implant technology.
Over the same time period that technology has been improving, the age at identification of hearing loss in infants has been reduced from an average of about 24 months to less than 6 months because of mandatory newborn hearing screening now adopted by most states of the US . Methods of determining the degree and type of hearing loss in infants have improved, as has identifying the etiology of hearing loss, fitting of hearing aids and implementation of early family-oriented intervention . Earlier diagnosis and improved early intervention facilitate development of spoken language with or without cochlear implantation .
Selection Criteria for Candidates for Cochlear Implants
Earlier, cochlear implants were only given to deaf children 2 years and older with no open set word recognition with hearing aids. Later, the FDA approved implantation of cochlear implants in infants as young as 12 months of age and in children 2 years or older with severe hearing loss. This means that children with unaided thresholds of 70 dB HL or greater and open set speech perception scores up to 30% correct are now candidates for cochlear implantation. Children [27-29] and adults  with better preimplant hearing achieve better speech recognition with a cochlear implant. The reason may be that individuals with more residual hearing have more intact auditory structures available for electrical stimulation. Aided hearing before cochlear implantation is likely to maintain the ability of the central auditory pathway to process speech information [Kral and Tillein, this vol, pp 89-108]. Very early use of hearing aids in children with residual hearing may be beneficial because it provides input to the auditory nervous system and acts as a bridge to provide auditory access to language until the child receives an implant.
The average age of cochlear implantation in children has decreased steadily because of earlier diagnosis and changes in selection criteria . Receiving an implant at a younger age has a documented advantage for language development . The critical age dividing better and poorer postimplant outcomes has changed with a decrease in the average age of implantation and has been variously reported [Sharma and Dorman, this vol, pp 66-88; Kral and Tillein, this vol, pp 89-108]. The critical age below which use of a cochlear implant results in no further benefit has yet to be determined, but preliminary data suggest that this may occur at about 12 months of age .
Measuring the effects of earlier age at implantation is complicated by the fact that children with more preimplant residual hearing are typically implanted at somewhat older ages in the USA. This is due partly to the FDA guidelines that set stricter standards for implant candidacy between 12 and 24 months of age and partly because of the difficulty of conducting valid hearing aid trials in infancy. As a result, cochlear implantation in children with greater residual hearing is frequently postponed until evidence is compelling that hearing aids will not be sufficient for optimum spoken language progress. This practice may mask the true benefits of earlier implantation, since children implanted at the youngest ages are more likely to exhibit bilateral profound deafness and may be predisposed to slower development of speech perception skills both prior to and following cochlear implantation than children with more residual hearing.
Because hearing aids are noninvasive and relatively inexpensive compared with a cochlear implant, the choice of a cochlear implant over a hearing aid requires justification. Audiologists and surgeons must advise parents regarding the benefit of a cochlear implant over an appropriately fitted conventional highpower hearing aid. In studies using early versions of cochlear implant technology, children with pure tone average (PTA) thresholds of 100 dB HL or greater have been shown to exhibit better speech discrimination with cochlear implants than with hearing aids, but no difference was found for children with PTA thresholds between 90-100 dB HL [14, 34, 35]. Superior performance of pedi-atric implant users with profound losses compared to children who used analog, linear amplification caused a change in the selection criteria to include children with more residual hearing . However, the question of what levels of residual hearing will result in better performance with a hearing aid or a cochlear implant must be regularly revisited as new developments in technology improve speech perception with both devices. Speech perception assessment of profoundly deaf children that is typically conducted in quiet at a relatively loud level (70 dB SPL) must be adapted to reflect the steadily changing capabilities of newer implants and hearing aids.
Advances in cochlear implant speech coding strategies have occurred at the same time that newer and better hearing aids with digital signal processing (DSP) circuitry and wide dynamic range compression have evolved. DSP hearing aids can improve word recognition scores, particularly for speech presented at low sound levels . Similarly, cochlear implant advances including use of higher sensitivity control settings for microphone input gain  and use of higher minimum stimulation levels  have resulted in improved perception of soft speech as well [37, 40]. Perception of soft speech may be critical for situations in which the child is not close to the talker. These situations are common in the typical language-learning environments of infants and young children, and it is important to compare the most recent generation of each sensory aid under these less than optimal listening conditions. A recent study that compared speech perception skills achieved by children who use the most up-to-date technologies  concluded that digital hearing aids and cochlear implants provide similar access to speech presented at conversational levels (~60dB SPL) or louder (70 dB SPL or higher). However, the implant offered an advantage for soft speech (e.g. 50 dB SPL) that increased as the severity of the loss and gain requirements increased. Assuming the importance of soft speech for incidental language learning, self-monitoring of speech, and ease of communication in a variety of 'real world' listening conditions, these results indicate that if a child is not making expected progress in developing spoken language with an optimally fit DSP hearing aid with wide dynamic range compression and the unaided PTA is greater than 92 dB HL, a cochlear implant may provide significantly more benefit than a hearing aid. Studies with children who have continuously used the most up-to-date cochlear implant or hearing aid technology from a young age are needed to confirm the impact of improved perception of soft speech and speech in noise on spoken language development.
Child, Family and Educational Characteristics Affecting
Characteristics of prelingually deaf children, their families and their educational settings may affect the rate of acquisition of spoken language following cochlear implantation. The impact of these factors on outcome scores may be controlled in research designs either by sample selection or by statistical techniques after data have been collected .
Girls exhibit a verbal advantage over boys in both hearing  and hearing-impaired  populations. This advantage was apparent in children with 4-6 years of cochlear implant use, where girls scored significantly higher than boys in measures of speech production, English language competence and reading skills .
Age at Onset/Duration of Deafness
While onset of deafness after birth is generally considered an advantage over congenital deafness for auditory development , when only those children with age at onset under 3 years of age are considered (i.e. prelingually deaf), the advantage of later onset was no longer apparent in speech perception outcomes [47, 48]. It was reported  that 80% of children who became deaf between birth and 36 months of age and received a cochlear implant within a year of onset of deafness achieved both speech and language skills within expected levels for hearing age-mates when they were 8 or 9 years old. Only 36% of children with similar age at onset of deafness but implanted after being deaf for 2-3 years achieved similar speech and language skills. In a group of congenitally deaf children, 43% of the children implanted at 2 years of age achieved both speech intelligibility and language skills commensurate with their hearing age-mates by early elementary school age compared to only 16% of those implanted at the age of 4 years. These studies show that children who have a shorter duration of unaided deafness, whether congenital or acquired, have the best chances to achieve results that fall within the range of hearing age-mates, for both speech and language levels.
With a few exceptions, the cause of deafness has not been found to predict postimplant outcomes in children [45, 50]. Naturally, individuals in whom deafness is caused by auditory nerve pathology will not benefit from cochlear implants [Colletti, this vol, pp 167-185; M0ller, this vol, pp 206-223]. These are few, and for the majority of congenitally deaf children the site of the problem is the cochlea although the exact cause is unknown but presumed to be genetic in origin. The most common cause of congenitally inherited sensorineural hearing loss in the Midwestern United States is mutations in GJB2 (gap junction protein p2), the gene that encodes for the gap junction protein Connexin 26. The predicted prevalence of GJB2-related SNHL is 22.7 per 100,000 births . A recent study  conducted Connexin 26 evaluations of 55 cochlear implants users and related their genetic status to their cognitive, language and reading outcomes. The 22 children who tested positive for Connexin 26 achieved significantly higher scores in reading comprehension and on a standardized block design task (a nonverbal cognitive measure) than those children who were Connexin 26 negative. These results suggest that the isolated insult to the cochlea created by Connexin 26 mutations allows for preservation of the auditory nerve in the cochlea and perhaps central cognitive function, which may explain the better outcome in children with Connexin 26-related hearing loss.
Family factors associated with spoken language progress in children with hearing loss include higher parent education and income [45, 53], parental involvement in linguistic development [54, 55], smaller family size , and intact family structure . The impact of these family factors may interact with educational factors, since children enrolled in oral-aural preschools tend to exhibit a more favorable family environment profile .
Children with motor and/or cognitive delays, including mental retardation, autism and cerebral palsy, are slower in their development of speech perception and language skills following cochlear implantation [50, 58, 59]; even children without a diagnosed additional disability exhibit a significant relation between nonverbal cognitive ability (i.e. Performance IQ) and postimplant outcomes .
Early development of auditory processing skills, including those associated with attention, learning and memory, may affect the spoken language outcomes achieved with a cochlear implant. The contribution of auditory processing and verbal working memory to linguistic achievements, such as vocabulary acquisition, has already been documented for hearing children [60, 61] and for deaf children before the advent of cochlear implants . It has been demonstrated that auditory memory, verbal rehearsal and serial scanning abilities are also important predictors of speech and language skills in children with cochlear implants . Performance of cochlear implant users has been compared with hearing age-mates on serial recall , working memory  and nonword repetition tasks . Typically, children with cochlear implants did not perform as well on these tasks as their hearing age-mates, either due to the lasting effects of early auditory deprivation or to the incomplete auditory signal provided by the implant. Correlations of scores on these cognitive measures with indices of speaking rate indicated that slower processing speeds might play a role in creating shorter memory spans. Nevertheless, some children with implants scored within or close to the average range for hearing children on tests of memory and processing speed, and these children exhibited the best speech and language outcomes  and were more likely to be enrolled in oral communication (OC) programs after implantation. This suggests that the auditory exposure that children receive after implantation can influence their auditory processing skills and thereby facilitate spoken language learning.
Most children with severe hearing loss receive early special education, beginning with individual family-oriented sessions and often continuing into preschool and elementary school classrooms. There is evidence to suggest that access to auditory information via a cochlear implant, especially if acquired at an early age, may decrease the time required for special education and thus result in an earlier entry into a mainstream education setting with hearing children . The proportion of children enrolled in mainstream classes has been shown to increase with each year of cochlear implant use . Earlier educational mainstreaming following cochlear implantation is associated with higher speech intelligibility  and better reading scores . These data belie the importance of early special intervention that may serve to prepare children for mainstreaming at a young age. With early cochlear implantation and special educational intervention by 2 years of age, many young deaf children are ready for mainstream school placement earlier, exhibiting speech and language skills that approach those of hearing age-mates by 5 or 6 years of age . Early educational mainstreaming may, in fact, be a result rather than a cause of good speech and reading skills in elementary school.
Whether children are enrolled in special education or mainstream classes, the communication mode used may influence postimplant spoken language outcomes. This variable is most often dichotomized into OC approaches and TC approaches. Proponents of the OC approach maintain that dependence on speech and audition for communication is critical for achieving maximum auditory benefit from any sensory aid. Constant use of the auditory system to monitor speech production and to comprehend spoken language provides the concentrated practice needed for optimum benefit from a cochlear implant. There is considerable evidence that children enrolled in OC programs have better speech perception, better speech production and their overall language improvement after implantation is better than those in TC programs [73-75]. This is why cochlear implants were first recommended for children enrolled in OC settings, but there has been a trend toward greater acceptance of cochlear implants by families and educators who use TC . Proponents of the TC approach maintain that the deaf child benefits most when signed English accompanies speech. The use of a sign language facilitates learning language through the use of nonauditory means. The child is then able to associate what he/she hears through the implant with signed representations of language supporting development of spoken language. The advantages of cochlear implants over hearing aids for increasing language competence in children enrolled in TC settings have been demonstrated many studies [77-79]. Some studies have documented faster vocabulary improvement following cochlear implantation for children enrolled in TC programs than those enrolled in OC programs, especially when children are implanted young [22, 80].
It is a challenge to determine the benefits of one method over another for postimplant spoken language development because the results may be affected by the chosen sample characteristics. For example, children implanted at younger ages are more likely to use OC exclusively than children implanted somewhat later. Thus, in a study of early implantation effects Holt et al.  found that 87.5% of their subjects implanted between 7 and 12 months of age used OC, while only 44.3% of those implanted between 37 and 48 months did so. The selection criteria used for participants in such studies can affect the outcome. Programs emphasizing spoken language may favor the admission of children with certain characteristics (e.g. greater preimplant residual hearing, higher family socioeconomic status, higher IQ) that are also associated with improved spoken language outcomes. Carefully controlled research is needed to determine whether the emphasis on spoken language provided in oral education settings really facilitates speech and language development with an implant, or whether children who make good progress with an implant get placed in oral settings, thus biasing the observed outcome.
A recent study that was statistically controlled for the effects of a variety of child, family and implant characteristics (including all intervening variables described above) examined the effects of various education and rehabilitation models on the deaf child's postimplant development . The 181 children included in this study were 8 or 9 years old, deaf before 3 years of age, implanted by age 5 and had used an implant for 4-7 years. Approximately half of the children came from OC and half from TC classrooms. These children thus did not represent any single program or method, but rather came from the range of educational settings available in North America, and children whose classrooms included greater emphasis on speech and auditory skill development exhibited significantly better speech perception , speech production [70, 81] and language  skills in early elementary school than children who had received more emphasis on sign language. The extent to which the child's educational program emphasized speech and audition accounted for a significant part of the variance in the outcomes, even after variance due to other education variables, including amount of individual therapy, school setting (public/private), and classroom type (mainstream/special education) was removed. A separate analysis of outcomes for the small number of children who had changed communication mode over the 5-year period following cochlear implantation indicated that children who were successful in acquiring speech in a TC environment tended to move into oral classrooms following cochlear implantation, further inflating the OC/TC difference . Children who depended exclusively on speech for communication had significantly better speech and language outcomes than children who used both speech and sign language.
Overall Language Achievements of Early Implanted Children
The vast and growing literature on the achievements of profoundly deaf children following cochlear implantation indicates a dramatic shift towards spoken language skills that closely approximate those of hearing children. These levels are unprecedented in previous studies of profoundly deaf children who used hearing aids. For example, in a nationwide study of 181 children in early elementary school who had been implanted before 5 years of age, half of the participants exhibited speech that was at least 80% intelligible to naive listeners  and 47% had age-appropriate spoken language skills . Over half (52%) of the children had age-appropriate reading scores  and 58% were fully mainstreamed into classes with their hearing age-mates, while another 23% were partially main-streamed . These results were achieved by a group of children with little or no preimplant residual hearing using early generations of cochlear implant technology (most children initially received Nucleus-22 implants with MSP processors that were later upgraded to SPEAK) [Loizou, this vol, pp 109-143]. The participants in the study had experienced a wide range of educational methods, including both OC and TC approaches and both special education and mainstream settings. None of these children had the benefit of cochlear implantation before 2 years of age. The reported performance levels may underestimate the potential achievements of more recent recipients of modern cochlear implant devices once such children have accumulated comparable implant experience.
Evidence from young children using newer versions of cochlear implant technology show that once they receive cochlear implants, their characteristically slow rate of language development accelerates and they start developing language at a near-normal rate. The developmental gap between deaf and hearing age-mates, which typically increases with age, remains about the same size (measured in units of language age) following cochlear implantation. If this pattern is maintained, congenitally deaf children might exhibit only a negligible delay in language development if they receive an implant early enough in life . Further research is needed to determine whether this normal rate of language acquisition extends to phonetic and phonemic development in speech production, and whether this growth rate continues as the children grow older and acquire complex language, vocabulary and literacy skills.
Many factors influence a child's ability to obtain benefit from a cochlear implant. The amount of benefit is a product of what the child brings to the learning environment, what is provided by the implant itself, and what is provided by the child's rehabilitation program. Our ability to influence intrinsic factors such as the child's intelligence or the family environment is limited. However, we can insure that each child gets the most up-to-date processing strategy with a well-fitted device at the youngest age that is practical, and we can help to provide each child with an emphasis on speech and auditory skill development both at home and in their educational program. Results indicate that attention to all these factors can make a significant difference in the overall benefit children obtain from the use of cochlear implants.
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Ann E. Geers, PhD 167 Rocky Knob Rd. Clyde, N.C. 28721 (USA) E-Mail [email protected]
M0ller AR (ed): Cochlear and Brainstem Implants.
Adv Otorhinolaryngol. Basel, Karger, 2006, vol 64, pp 66-88
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