Central auditory system activity and development in children who use cochlear implants

Infants with early-onset severe to profound deafness are deprived of auditory stimulation until they receive a cochlear implant. Auditory input is achieved by electrical stimulation of the auditory nerve. I asked: (1) Does early-onset deafness impact the central auditory pathways? (2) Will the duration of early-onset deafness affect the system's ability to respond to electrical stimulation? (3) Can the pathways change in response to chronic stimulation? and (4) If plasticity exists, is it limited by the age at implantation?; Electrically evoked potentials of the auditory nerve (ECAP), brainstem (EABR) and thalamo-cortex (EMLR), were collected repeatedly in 50 children with early-onset deafness (5.4 +/- 4.0 years); recordings were made at implant surgery, initial device activation and at regular times over the first year of implant use. Responses were also collected from 31 children (6.3 +/- 3.0 years at implantation, 5.3 +/- 2.9 years of implant experience) and in 11 adults (implanted at 42.5 +/- 7.5 years, 4.5 +/- 2.9 years of implant use).; In answer to the questions posed, I found, first, that most children at initial device activation had recognizable ECAP and EABR responses (89% and 94%, respectively) but only 32% had detectable EMLRs. Second, initial ECAP and EABR wave latencies and amplitudes had no significant relationship with age at implantation whereas EMLR detectability was better, wave latencies shorter, and wave amplitudes larger in children implanted at ≥5 years compared to their younger peers. Thus, early-onset deafness and its duration influence thalamocortical responses but not auditory nerve or brainstem responses. Third, significant decreases in latencies and increases in amplitudes of all responses and increased EMLR detectability were found with ongoing implant use reflecting activity-dependent changes along the central auditory pathways promoted by implant use. Fourth, the ECAP and EABR changes were not significantly related to the age at implantation and showed some similarities to changes in the normal acoustically evoked brainstem response. Increases in EMLR detectability tended to be more subtle in children implanted between 8--17 years than changes in younger children. Age and/or duration of deafness therefore affect thalamo-cortical responses more strongly than responses from more peripheral areas.