The Effects of Asymmetric Hearing on Bilateral Brainstem Function: Findings in Children with Bimodal (Electric and Acoustic) Hearing

Abstract

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.

Limiting asymmetric hearing improves benefits of bilateral hearing in children using cochlear implants

“…By having better residual hearing and access to sound than previous bimodal users 46 , the present cohort of bimodal users exhibited improvement in speech perception and detection on par with their peers using two CIs (Figs 4b and 6a). Like bilateral CIs, bimodal devices adequately stimulate the bilateral auditory pathways to promote symmetric brainstem development 13 and typical hemispheric representation of sound 27,28 when provided with limited delay to children with sufficient residual hearing. For the deaf ear, a CI outperforms a HA: the 30.5 ± 23.8 RAU bilateral benefit over listening with a HA alone (Fig.…”

“…Notably, electrophysiological measures similarly assert an underlying premise for aural preference that emphasizes the importance of minimizing delay to bilateral hearing in bimodal and bilateral implant users. Extensive reorganization of auditory pathways occurs with asymmetric hearing during development, leading to an over-representation of the better ear 13,21,23,24,27–31,69 . Brainstem asymmetries 69 and cortical aural preference 21,28 favouring the first hearing/implanted ear occur within 2–3 years of asymmetric input.…”

“…We thus sought to promote bilateral hearing development by providing the most appropriate device in each ear to our large cohort of children with deafness 7 . A cochlear implant was provided in ears with severe/profound deafness; children with bilateral deafness received two cochlear implants whereas children with better hearing in one ear received one cochlear implant and a hearing aid in the other, better hearing, ear (bimodal hearing) 7,11–13 .…”

“…Whereas electrophysiological data included children with a range of hearing in the non-implanted ear (normal to severe/profound) 13,27,28 , previous studies largely focused on children who have significant (severe/profound) hearing loss in their non-implanted ear 46–50 . These studies reveal benefits of bimodal hearing over the use of a cochlear implant alone but continued challenges for listening to speech in noise.…”

Bilateral cochlear implants in children: Effects of auditory experience and deprivation on auditory perception

“…Development is driven preferentially from the unilaterally stimulated ear [40], compromising responses from the opposite pathways and potentially disrupting binaural hearing. Efforts to avoid these changes include simultaneous bilateral implantation [41–44], and the use of a hearing aid in the unimplanted ear (bimodal device use) [45–48]. …”