The early years of a child’s life are critical for acquiring information about the world and developing a cognitive foundation to learn language (Tobey, Geers, Brenner, Altuna & Gabbert, 2003). Exposure to speech and language during early years of life has a lasting impact on a child’s auditory, speech, language and literacy development (Connor, Craig, Raudenbush, Heavner & Zwolan, 2006). Research has shown that children who are deprived of this input have associated deficits in language development, reading, verbal intelligence and social-emotional development (Nicholas & Geers, 2006). Clearly, it is to the advantage of every child to form a solid foundation in language.
Children with profound sensorineural hearing loss are at significant risk for serious speech and language delays that can impact their communication, academic and social development (Connor et al., 2006). One treatment method for this degree of loss is the use of a cochlear implant. A cochlear implant can aid in the development of language skills and has been associated with stronger results in speech perception, speech production, language and reading (Tobey et al., 2003). A central issue in the field of cochlear implants is whether there is an optimal age range for implanting a deaf child (Sharma, Dorman & Spahr, 2002). This is known as the critical or sensitive period hypothesis for speech and language development. This theory suggests that delaying a child’s cochlear implant could jeopardize their lifetime communication abilities by limiting critical auditory information needed for development (Tobey et al., 2003). During early development the central auditory system learns to organize sensory input (Sharma, Dorman & Kral, 2005). If that developmental period has passed without hearing experience, widespread degeneration of the central auditory system will occur (Sharma et al., 2002). Early cochlear implantation is crucial for the development of language in congenitally deaf children due to a sensitive period where the central auditory pathway is maximally plastic (Sharma, et al., 2005).
In the documentary Sound & Fury (Weisberg & Aronson, 2000), Heather’s parents decide to withhold cochlear implantation until she is old enough to make the decision for herself. However, because there is such a strong emphasis on early implantation, Heather’s parents are in fact depriving her of critical auditory input needed for speech and language development. This paper will discuss three clinical research studies that provide information about sensitive periods, factors that lead to deterioration of the central auditory pathways and characteristics of plasticity in a developing brain.
Research on sensitive periods and age-related plasticity is commonly used when describing different aspects of neurosensory development (Kral & Eggermont, 2007). Physiological studies of sensory development and function have shown that the plasticity of central auditory pathway is greater during early development than in mature development (Harrison, Gordon & Richard, 2005). For congenitally deaf children, a cochlear implant during early years can provide sufficient auditory information to aid in the development of speech and language (Tobey et al., 2003). To better understand the sensitive period of development, researchers Harrison et al. (2005) from the Hospital for Sick Children in Toronto investigated the early development of human auditory function through different measures of auditory function and speech understanding. The experiment consisted of 82 congenitally deaf children who had received a cochlear implant between the ages of two to thirteen years (Harrison et al., 2005). All of the children had attended the Cochlear Implant Program at the Hospital for Sick Children and had used their implant for at least five years. The children were grouped together by age of implantation and follow ups were made eight years postimplant. The measures that they used to assess the children were standardized speech perception tests that included the Test of Auditory Comprehension and the Glendonald Auditory Screening Procedure.
The researchers found that the children who were implanted at six years of age or younger scored higher than children implanted at older ages (Harrison et al., 2005). The children who were implanted at eight years of age or later did not perform well and exhibited very poor performance on each test. This means that the children who had the shortest duration of auditory deprivation performed better on speech perception tests compared to those who were implanted later in childhood (Harrison et al., 2005). By six years of age the younger children had outperformed their older peers in all phoneme and word speech perception tasks. It was also found that the children who were implanted at two years of age actually exceeded all other age groups even when compared to those with longer periods of implant use.
The results of the Harrison et. al. (2005) study suggest that speech perception outcomes are related to age at implantation or duration of auditory deprivation. The study also informs us about age-related auditory system plasticity. This is evidence that there is a sensitive period in development where auditory input must be received for better speech and language development, and that central auditory plasticity is limited for children who are implanted at an older ages (Harrison et al., 2005). For the family in Sound & Fury (Weisberg & Aronson, 2000), this is of additional importance because the existence of a sensitive period may influence the parents’ decision about clinical intervention and why it is so important for early implantation.
Research on the effects of auditory deprivation suggest the best time to implant a child is before sensory deprivation alters the development and plasticity of the central auditory system (Sharma et al., 2002). It is hypothesized that children who experience a period of deafness of seven years or more before implantation never fully develop a functional network of central auditory pathways needed for language (Sharma et al., 2005). In an attempt to test the sensitive period hypothesis, Sharma et al. (2002) investigated the consequences of cochlear implantation at different ages and the effects on the central auditory system. In order to measure the maturity of the central auditory pathways, researchers recorded cortical auditory evoked potentials (CAEP) from 136 normal-hearing participants from 0.1 to 20 years old, and from 104 congenitally deafened cochlear implant users from 2.3 to 18 years old (Sharma et al., 2002). Recordings from the implant users were made at least six months after the device was switched on. To compare brain development of the implant users they were divided into three groups: early, middle and late implantation. The early group had been implanted by 3.5 years of age, the middle group was implanted between 3.6 and 6.5 years of age, and the late group was implanted after 7 years of age.
Testing took place in a sound booth where the older children were seated comfortably in a reclining chair and the younger children were seated on their parent’s lap (Sharma et al., 2002). Cortical auditory evoked responses were recorded in response to a synthesized speech syllable /ba/ presented for 90 seconds at starting frequencies of F1 and F2 at 234 Hz and 616 Hz, respectively. The stimulus was delivered via a loud speaker placed at an angle of 45° to the right of normal-hearing subjects, and for the implanted subjects the speaker was moved to the side of their implant (Sharma et al., 2002).
The researchers examined the P1 response latencies in participants fit with cochlear implants in the early, middle and late age groups (Sharma et al., 2002). P1 latency can be used to understand how mature the auditory pathways are in congenitally deaf children who regain hearing after cochlear implantation (Sharma et al., 2005). It was found that the deaf children in the early implanted group demonstrated age-appropriate P1 latencies six months after implantation and the late implanted group had abnormal P1 latencies (Sharma et al., 2002). This suggests that auditory deprivation of more than seven years can significantly alter the P1 latencies in response to sound. Sharma et al. (2002) concluded that there is a definite sensitive period during early development of approximately 3.5 years where the auditory system is highly plastic. However, it was noted that for a small number of children the central auditory pathways may remain plastic for up to seven years due to factors such as duration of hearing-aid use (Sharma et al., 2002).
The results of this study show why it is so critical Heather is implanted at an earlier age (Weisberg & Aronson, 2000). At 6-years-old she is at the cusp of the sensitive period and postponing her implant any longer could result in irreversible degeneration of the central auditory system (Sharma et al., 2005). The lack of auditory input she is receiving is causing the auditory pathways to become non-adaptive and she could potentially lose the ability to learn language (Nicholas & Geers, 2006).
Researchers interested in neurobiological brain development suggest that certain areas of the cortex will re-organize if stimulation is withheld for long periods of time (Sharma, Nash & Dorman, 2009). Once this reorganization occurs it cannot be restored back to normal (Sharma et al., 2005). In the auditory system stimulation must be delivered within a narrow window of time in order for it to develop normally (Kral & Eggermont, 2007). To demonstrate this, Sharma et al. (2009) presented two clinical cases of congenitally deaf children fitted with cochlear implants to describe the existence of a sensitive period for the development of the central auditory pathways.
In Case 1, a 10-year-old child was diagnosed with profound hearing loss after failing an infant hearing screening (Sharma et al., 2009). He was fitted with hearing aids at 4 months which he used until he received his cochlear implant at 1.4 years of age, an age well within the sensitive period for central auditory development. The CAEP recorded from this child showed an age-appropriate P1 latency. He also scored 92% on the Lexical Neighborhood Test of speech perception, which is considered excellent for an implanted child his age. The results suggest that early implantation within the sensitive period can promote age-appropriate language development, if early auditory stimulation is available (Sharma et al., 2009). This is a prime example of the benefits of early cochlear implantation.
In Case 2, a 2-month-old child was diagnosed with severe-to-profound hearing loss after losing it from meningitis (Sharma et al., 2009). Her primary mode of communication was sign language, which she used in early childhood. At age four she was fitted with hearing aids, but it was reported that she did not wear them consistently. Finally at age 7.4 she received a cochlear implant, an age that falls beyond the sensitive period for central auditory development. The CAEP recorded from this child showed an abnormally developing auditory cortex, which typically occurs after the end of the sensitive period. Also, her performance on speech perception tests was minimal. This shows that late implantation after the sensitive period can result in poor language outcomes (Sharma et al., 2009).
There are many possible reasons why this child showed irregular P1 latency. Firstly, she was deprived of sound for a long period of time. Not receiving hearing aids until the age of 4 and inconsistently using them does not provide her with sufficient auditory stimulation, which is critical for the development of the central auditory pathways (Sharma et al., 2009). Secondly, if there is inadequate sensory input during early years of development it can cause irreversible degeneration of the central auditory system (Sharma et al., 2005). By the time she was implanted, neurological changes had already occurred and her brain was minimally plastic, unable to adapt to the cochlear implant.
The results from these two cases suggest the best time to implant a congenitally deaf child with a cochlear implant is within the first 3.5 years of life when the central auditory pathways are highly plastic. After the sensitive period ends at around age seven, the effectiveness of a cochlear implant is significantly reduced (Sharma et al., 2009). In Sound & Fury (Weisburg & Aronson, 2000), Heather’s parents should be aware that postponing her cochlear implant is causing the central auditory pathways to re-organize. Since there is no auditory stimulation available, it will be very hard for Heather to develop language if she decides to pursue cochlear implantation at a later age. On the other hand, given the early implantation of baby Peter at age one, it possible to expect good behavioural outcomes for this child, including age-appropriate language development.
In conclusion, evidence from clinical studies of cochlear implantation suggests that there is a sensitive period of development, ending around seven years of age when the central auditory pathway is maximally plastic (Sharma et al., 2002). It was shown that children who were implanted at six years of age or younger scored better on standardized speech perception tests compared to children implanted after six years of age (Harrison et al., 2005). This demonstrates that auditory deprivation longer than six years can alter the development and plasticity of the central auditory system. Also, congenitally deaf children who regained hearing after cochlear implantation demonstrated age-appropriate P1 latencies six months after implantation if implanted during the sensitive period (Sharma et al., 2002).
Children who receive effective auditory stimulation during the sensitive period have the potential to develop near normal speech and language (Sharma et al., 2009). In contrast, congenitally deaf children implanted after seven years of age exhibit incomplete maturation of the central auditory pathway and limited neural plasticity, which is crucial for the auditory cortex to adapt to the implant. Clearly, cochlear implant intervention at an early age in congenitally deaf children can result in significantly better outcomes in speech and language development and should be implemented in all deaf children. In Sound & Fury (Weisberg & Aronson 2000), Heather’s parents are making a major mistake by dismissing the cochlear implant, which is needed to make a lasting impact on a her speech and language development.
References
Connor, C. M., Craig, H. K., Raudenbush, S. W., Heavner, K., Zwolan, T. A. (2006). The age at which young deaf children receive cochlear implants and their vocabulary and speech-production growth: Is there an added value for early implantation? Journal of Ear and Hearing, 27, 628-644.
Harrison, R. V., Gordon, K. A., Richard, J. M. (2005). Is there a critical period for cochlear implantation in congenitally deaf children? Analyses of hearing and speech perception performance after implantation. Wiley Periodicals, 46, 252-261.
Kral, A., Eggermont, J. (2007). What’s to lose and what’s to learn: Development under auditory deprivation, cochlear implants and limits of cortical plasticity. Brain Research Reviews, 56, 259-269.
Nicholas, J. G., Geers, A. E. (2006). Effects of early auditory experience on the spoken language of deaf children at 3 years of age. Journal of Ear & Heaingr, 27 (3), 286-298.
Sharma, A., Dorman, M. F., Kral, A. (2005). The influence of a sensitive period on central auditory development in children with unilateral and bilateral cochlear implants. Hearing Research, 203, 134-143.
Sharma, A., Dorman, M., Spahr, A. J. (2002). A sensitive period for the development of the central auditory system in children with cochlear implants: Implications for age of implantation. Journal of Ear and Hearing, 23, 532-539.
Sharma, A., Nash, A. A., Dorman, M. (2009). Cortical development, plasticity and re-organization in children with cochlear implants. Journal of Communication Disorders, 42, 272-279.
Tobey, E. A., Geers, A. E., Brenner, C., Altuna, D., Gabbert, G. (2003). Factors associated with development of speech production skills in children implanted by age five. Journal of Ear and Hearing, 24, 36-45.
Weisberg, R. (Producer), & Aronson, J. (Producer/Director). (2000). Sound and fury [Motion picture]. United States: Aronson Film Associates and Public Policy Productions.
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