PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Benefits and challenges of bimodal hearing in children
Melissa Jane POLONENKO1, 21

University of Rochester Medical Center, United States
2

The Hospital for Sick Children, Canada

ABSTRACT
Binaural hearing is important for listening in and navigating everyday acoustic environments. Unilateraldeafness during development drives extensive reorganization in bilateral auditory pathways, limiting spatial
hearing and putting children at risk for social and educational challenges. In an effort to reduce these listening
challenges and support bilateral development, implantation criteria are expanding to provide children who
have asymmetric hearing loss a cochlear implant for the deaf ear and a hearing aid to the other ear whenneeded, known as bimodal hearing. Our work has assessed hearing development in bimodal users throughevoked potentials and behavioural measures of spatial hearing and speech perception. This talk will discuss
how bimodal hearing can promote symmetric bilateral development but faces limitations for preventing orreversing asymmetry-driven reorganization when implantation is delayed or residual hearing is insufficient.
Challenges still faced by children who hear bimodally will also be discussed.
Keywords: Cochlear implant, Development, Outcomes

1. INTRODUCTION
Children spend much of their time in complex acoustic environments, in which they need to be able
to hear where different sounds are coming from and pick out different voices in order to listen. Theseenvironments can be challenging, with multiple competing sounds. To localize sound and listen innoise, the auditory system compares the timing and level differences of sound reaching the two ears,
called binaural hearing (1). However, if children have a deficit in one or both ears, these cues are
distorted, and spatial hearing is compromised. Not only do issues with spatial hearing make listening
in noise harder, but they put children at risk of challenges to langu age acquisition, socialization andacademic progress (2–5). Indeed, periods of unilateral hearing during key developmental periods drive
reorganization of bilateral auditory pathways through strengthening of pathways from the hearing ear
and relative weakening of pathways from the deaf ear (6–9). These neurophysiological changes createa system that remains preferential to information from the only/better hearing ear, called “auralpreference”, that distorts and disrupts binaural representation of sound and compromises binauralhearing (10). Therefore, it is important to consider the binaural system when deciding on treatment ofhearing loss. Consequently, it is now recommended to provide children with bilateral hearing as earlyas possible with the most appropriate device(s) for the hearing loss in each ear (10).
Both cochlear implants (CI) and hearing aids are auditory prostheses that restore access to soundfor children with hearing loss. Some children benefit from a hearing aid if their residual hearing issufficient, but when the hearing loss is too severe a cochlear implant can electrically stimulate theauditory nerve to provide sensation of sound. In line with the recommendation to provide bilateraldevices early, it has become standard practice at many centres around the world to give two cochlear
implants to children who are bilaterally deaf. If both cochlear implants are provided early and withshort to no delay between surgeries, then symmetric cortical development occurs (8). Otherwise, if
children experience unilateral stimulation with one cochlear implant for more than two years before
they receive a second implant, unilaterally driven aural preference develops and persists despite yearsof subsequent bilateral implant use. As candidacy criteria are expanding to include children who haveasymmetric hearing with residual hearing in the better ear, bilateral hearing is provided throughbimodal hearing – electrical hearing with a cochlear implant to the deaf ear and acoustic hearing witha hearing aid in the better hearing ear with residual hearing.
1

mpolonenko@gmail.com

2249

2. HEARING DEVELOPMENT IN CHILDREN WITH BIMODAL HEARING
We assessed hearing development in bimodal users by measuring changes in the brainstem andcortex, as well as measuring how children hear speech and important differences in sound between
their ears. Here we quickly review the benefits and challenges of bimodal hearing for promoting
bilateral auditory development in children.
2.1 Benefits of bimodal hearing
Electrical and acoustic hearing are unique and stimulate the auditory system in different ways .
Despite this, bimodal hearing promoted symmetric development of the bilateral auditory pathways forchildren who had sufficient residual hearing and received an implant with limited delay. These
children with residual hearing exhibited symmetric neural conduction in bilateral brainstem pathways,
as measured by similar wave III-V interwave intervals on each side (11). In the auditory cortices,
expected contralateral representation of each ear rapidly developed within six to ten months of implantuse (12,13) and consistently maintained this expected organization after two years of bimodal hearin g
experience (14). Moreover, most children experienced a bilateral advantage for speech perception overwearing either device alone (15), benefited from spatial hearing (15) and could detect changes inbinaural inter-aural level cues (11). These findings suggest that the auditory system can use different
modalities for bilateral hearing.
2.2 Challenges of bimodal hearing
However, there are limitations to the ability of bimodal hearing to promote symmetric bilateralauditory development. Various degrees of asymmetric hearing loss drove reorganization within theauditory system that resulted in an aural preference for the better hearing ear (12–14). This indicatesthat the developing auditory system is sensitive to imbalances in excitation and inhibition coming fromeach ear. Remarkably, asymmetrically driven reorganization followed a similar time course asunilaterally driven reorganization (14). These changes could be reversed if a cochlear implant wasprovided quickly (12), but longer delays to implantation coupled with poorer hearing in the worse
hearing ear limited neuroplastic changes upon implantation (13). Furthermore, longer periods withgreater degrees of asymmetric hearing limited the advantage afforded by wearing both the cochlear
implant and the hearing aid over listening with the better ear alone (15). Children with bimodal hearing
may experience further challenges for binaural hearing due to the large timing differences in deliveryof electro-acoustic stimulation to the auditory system (11,16) and the independent processing
strategies employed in their two separate devices (17). These issues may distort binaural timing andlevel cues that are important for spatial hearing and listening in noise, and may further contribute tochallenges faced by children listening through bimodal devices.

3. CONCLUSIONSFor children with sufficient residual hearing in their better hearing ear, bimodal hearing promotes
cortical development and benefits speech perception and spatial hearing. However, prolonged
experience with asymmetric hearing while waiting for a cochlear implant extensively reorganizes theauditory system, limiting future efforts to restore bilateral and spatial hearing with bimodal devices.
These findings testify to the remarkable ability of the auditory system to use whatever input it receives
to help process and interpret the sounds occurring in daily life. Future advances in device
programming may help overcome peripheral and processing challenges exper ienced by bimodal users,
which may in turn improve their binaural hearing abilities.

ACKNOWLEDGEMENTS
This work was completed during my doctoral studies with Dr. Karen Gordon and Dr. Blake Papsin
at the Hospital for Sick Children and The University of Toronto. Funding was provided by theCanadian Institutes of Health Research (CIHR), The Hospital for Sick Children Research Institute, andThe University of Toronto.

REFERENCES
1.

2250

Grothe B, Pecka M. The natural history of sound localization in mammals--a story of neuronalinhibition. Front Neural Circuits. 2014;8:116.
2.

Lieu JEC, Tye-Murray N, Karzon RK, Piccirillo JF. Unilateral hearing loss is associated with worse

speech-language scores in children. Pediatrics. 2010 Jun;125(6):e1348-1355.
3.

Sangen A, Royackers L, Desloovere C, Wouters J, van Wieringen A. Single-sided deafness affects

language and auditory development - a case-control

study. Clin Otolaryngol Off J ENT-UK Off J Neth

Soc Oto-Rhino-Laryngol Cervico-Facial Surg. 2017 Jan 7;
4.

Yoshinaga-Itano C. From Screening to Early Identification and Intervention: Discovering Predictors

to Successful Outcomes for Children With Significant Hearing Loss. J Deaf Stud Deaf Educ. 2003;8(1):11–30.
5.

Yoshinaga-Itano C, Baca RL, Sedey AL. Describing the trajectory of language development in thepresence of severe-to-profound hearing loss: a closer look at children with cochlear implants versus hearing
aids. Otol Neurotol Off Publ Am Otol Soc Am Neurotol Soc Eur Acad Otol Neurotol. 2010
Oct;31(8):1268–74.
6.

Kral A, Heid S, Hubka P, Tillein J. Unilateral hearing during development: hemispheric specificity inplastic reorganizations. Front Syst Neurosci. 2013;7:93.
7.

Kral A, Hubka P, Heid S, Tillein J. Single-sided deafness leads to unilateral aural preference within

an early sensitive period. Brain. 2013 Jan;136(1):180–93.
8.

Gordon KA, Wong DDE, Papsin BC. Bilateral input protects the cortex from unilaterally-driven

reorganization in children who are deaf. Brain. 2013 May;136(5):1609–25.
9.

Popescu MV, Polley DB. Monaural deprivation disrupts development of binaural selectivity inauditory midbrain and cortex. Neuron. 2010 Mar 11;65(5):718–31.
10.

Gordon KA, Henkin Y, Kral A. Asymmetric Hearing During Development: The Aural Preference

Syndrome and Treatment Options. Pediatrics. 2015 Jul;136(1):141–53.
11.

Polonenko MJ, Papsin BC, Gordon KA. The effects of asymmetric hearing on bilateral brainstem

function: findings in children with bimodal (electric and acoustic) hearing. Audiol Neurootol. 2015;20
Suppl 1:13–20.
12.

Polonenko MJ, Gordon KA, Cushing SL, Papsin BC. Cortical organization restored by cochlear

implantation in young children with single sided deafness. Sci Rep. 2017 Dec 4;7(1):16900.
13.

Polonenko MJ, Papsin BC, Gordon KA. Cortical plasticity with bimodal hearing in children withasymmetric hearing loss. Hear Res. 2018 Feb 17;
14.

Polonenko MJ, Papsin BC, Gordon KA. Delayed access to bilateral input alters cortical organizationin children with asymmetric hearing. NeuroImage Clin. 2018;17:415–25.
15.

Polonenko MJ, Papsin BC, Gordon KA. Limiting asymmetric hearing improves benefits of bilateral

hearing in children using cochlear implants. Sci Rep. 2018 Sep 4;8(1):13201.
16.

Zirn S, Arndt S, Aschendorff A, Wesarg T. Interaural stimulation timing in single sided deaf cochlear

implant users. Hear Res. 2015 Oct;328:148–56.
17.

Wiggins IM, Seeber BU. Effects of dynamic-range compression on the spatial attributes of sounds innormal-hearing listeners. Ear Hear. 2012 Jun;33(3):399–410.

2251