# Binaural Fusion Sharpens on a Scale of Octaves During Pre-adolescence in Children with Normal Hearing, Hearing Aids, and Bimodal Cochlear Implants, but not Bilateral Cochlear Implants

## TL;DR

This study shows that children with normal hearing and some hearing devices improve their ability to process sounds from both ears, but this improvement doesn't happen for children with bilateral cochlear implants.

## Contribution

The study reveals distinct developmental trajectories of binaural pitch fusion in children with different hearing conditions and devices.

## Key findings

- Binaural pitch fusion sharpens by 1–3 octaves in children with normal hearing, bilateral hearing aids, and bimodal cochlear implants during pre-adolescence.
- Children with bilateral cochlear implants do not show narrowing of binaural fusion ranges with age.
- Interaural pitch discrimination remains stable across age and group differences.

## Abstract

The breadth of binaural pitch fusion, the integration of sounds differing in frequency across the two ears, can limit the ability to segregate and understand speech in background noise. Binaural pitch fusion is one type of central auditory processing that may still be developing in the pre-adolescent age range. In addition, children with hearing loss potentially have different trajectories of development of central auditory processing compared to their normal-hearing (NH) peers, due to disruption of auditory input and/or abnormal stimulation from hearing devices. The goal of this study was to measure and compare binaural pitch fusion changes during development in children with NH versus hearing loss and different hearing device combinations. Interaural pitch discrimination abilities were also measured to control for pitch discrimination as a potential limiting factor for fusion that may also change during development.

Baseline measurements of binaural pitch fusion and interaural pitch discrimination were conducted in a total of 62 (22 female) children with NH (n = 25), bilateral hearing aids (HA; n = 10, bimodal cochlear implants (CI; n = 9), and bilateral CIs (n = 18), with longitudinal follow-up for a subset of participants (18 NH, 9 HA, 8 bimodal CI, and 15 bilateral CI). Age at the start of testing ranged from 6 to 10 years old, with a goal of repeated measurements over 3–6 years. Binaural pitch fusion ranges were measured as the range of acoustic frequencies (electrodes) presented to one ear that was perceptually fused with a single reference frequency (electrode) presented simultaneously to the other ear. Similarly, interaural pitch discrimination was measured as the range of frequencies (electrodes) that could not be consistently ranked in pitch compared to a single reference frequency (electrode) under sequential presentation to opposite ears.

Children with NH and HAs initially had broad binaural pitch fusion ranges compared to adults. With increasing age, the binaural fusion range narrowed by 1–3 octaves for children with NH, bilateral HAs, and bimodal CIs, but not for children with bilateral CIs. Interaural pitch discrimination showed no changes with age, though differences in discrimination ability were seen across groups.

Binaural fusion sharpens significantly on the scale of octaves in the age range from 6 to 14 years. The lack of change in interaural pitch discrimination with increasing age rules out discrimination changes as an explanation for the binaural fusion range changes. The differences in the trajectory of binaural fusion changes across groups indicate the importance of hearing device combination for the development of binaural processing abilities in children with hearing loss, with implications for addressing challenges with speech perception in noise. Together, the results suggest that pruning of binaural connections is still occurring and likely guided by hearing experience during childhood development.

The online version contains supplementary material available at 10.1007/s10162-025-00975-4.