From silence to sounds : Outcomes of cochlear implantation in Finnish adults and children

Individuals with profound deafness have damages in the inner and outer hair cells in the cochlea. For this reason they make only little use of frequency information and thus have minimal capacities to perceive speech sound cues. Accordingly, speech perception without augmentative means is impossible for them. Profoundly deaf children have been deprived of sound from very early stages of their lives, which inevitably has an effect on linguistic development. Cochlear implants (CIs) have proved their advantage in compensating auditory deficiency both in adults and children with profound hearing loss. In Helsinki University Hospital cochlear implantations started in the early 1980s for 10 Finnish adults with profound deafness. Since that time, due to the fast development of speech processing strategies introduced to CIs, high frequencies and temporal features important to the discrimination of consonantal cues and understanding continuous speech are now often available. The up-to-date worldwide estimation of adults using CIs is 200 000, and the respective number of children is 80 000. In Finland the estimated number of children with CIs is approximately 300, and 500 adults, respectively. In Studies I and II we followed the development of MMN responses to vowel and frequency stimuli in postlingually deafened adults. Plasticity of auditory pitch discrimination driven by cochlear implant (CI) use during a 2.5-year follow-up was indicated both by an enhancement of the amplitude of mismatch negativity (MMN) event-related brain potential (ERP) to vowel contrasts as well as to pure tone frequency changes. The MMN was observed in all patients after 2.5 years of CI use. It was first seen for the larger vowel difference /e/-/o/ and later for the smaller one /e/-/o/. A significant enhancement was observed for the MMN elicited by 3200 Hz deviant tones among 4000 Hz standards. Instead, no observable mean amplitude MMNs were seen in duration stimuli. The MMN results were compared to audiometric speech recognition scores (SRSs) which improved over time. Participants with the highest SRSs had the highest MMN amplitudes. Studies III (n = 92) and IV (n = 164) report auditory and spoken language development as well as educational settings for Finnish children with cochlear implants. The results of the 92 children show that after approximately three years of hearing experience (hearing age = HA) at the mean chronological age of seven years, children are achieving developed forms of spoken language. The favourable age of two to three for early cochlear implantation in respect to good spoken language skills development was observed in 55 children. For Study IV, two questionnaires were employed; the first, concerning day care and educational placement, was filled in by professionals for rehabilitation guidance, and the second, evaluating language development (categories of auditory performance, spoken language skills, and main modes of communication), by speech and language therapists in audiology departments. Categories of auditory performance and spoken language levels were observed to grow in relation to age at cochlear implantation (p < 0.001) as well as in relation to proportional hearing age (p < 0.001). The composite scores for language development moved to more diversified ones in relation to increasing age at cochlear implantation and proportional hearing age (p < 0.001). Furthermore, the results also indicated that nearly half of the children were enrolled in normal kindergartens and 43% of school-aged children in mainstream schools. Children without additional disorders outperformed those with additional disorders. The results of Study IV indicate that the most favourable age for cochlear implantation could be about the age of two. Compared to other children, spoken language evaluation scores of those with additional disabilities were significantly lower; however, these children showed gradual improvements in their auditory perception and language scores. Taken together, the positive effects of cochlear implantation were observed in MMN responses in adults and in behavioral results for children with CIs.