The effects of asymmetric hearing on bilateral brainstem function: findings in children with bimodal (electric and acoustic) hearing.

As implantation criteria are broadening to include children with asymmetric hearing loss, it is important to determine the degree of residual hearing needed to protect the bilateral auditory pathways for binaural hearing and whether there is a sensitive period in development for implantation in these children. We have been studying these questions in a growing cohort of children. In the present study, auditory brainstem responses were recorded in 21 children who had 2.2 ± 2.2 years of bimodal hearing. Responses were evoked by 11-Hz acoustic clicks presented to the non-implanted ear and with biphasic electric pulses presented to the implanted ear. Twelve of these children also completed a behavioural task in which they were asked to which side of their heads bilaterally presented clicks/pulses that varied in interaural level or timing lateralized. All children experienced a delay in the non-implanted ear that resulted in 2.0 ± 0.35 ms longer peak latencies. These were further prolonged in 7 children as measured by longer interwave latencies from this ear than from the implanted ear. Despite large asymmetries in timing of brainstem activity between the two ears, all children perceived changes in interaural level differences. They were unable to detect differences in interaural timing cues. Symmetric brainstem function suggests bilateral development was preserved in some children. Future work will explore whether these children have better potential for developing binaural hearing using bimodal input.