# Neuronal mechanisms underlying binaural performance with cochlear implant in single-sided deafness: a [15O]water PET study

## TL;DR

This study uses PET scans to explore how the brain processes sound in people with cochlear implants for single-sided deafness, comparing those with good and poor performance.

## Contribution

The study identifies specific neuronal mechanisms linked to binaural integration and performance in SSD cochlear implant users using [15O]water PET.

## Key findings

- Poor-performing SSD-CI users showed reduced word understanding with unilateral stimulation and lacked left STG activation.
- Good-performing SSD-CI users and NH controls exhibited a significant asymmetry index change in PAC rCBF during bilateral stimulation.
- CI input to the PAC was comparable in both SSD-CI groups, but binaural integration was compromised in poor performers.

## Abstract

We examined neuronal activation patterns in single-sided deaf (SSD) cochlear implant (CI) recipients and normal-hearing (NH) controls to unravel neuronal mechanisms underlying binaural integration in SSD-CI.

20 SSD-CI (CI left) and 10 NH controls underwent [15O]water PET (assessing relative cerebral blood flow, rCBF) during auditory stimulation (forward or time-reversed, unilaterally to NH ear or bilaterally). Subjects were divided into SSD-CI with good (≥ 80%) and poor (< 80%) performance based on a median split of correct word understanding in PET background noise. Voxel-wise analyses identified volumes of interest for regional analyses (right primary auditory cortex (PAC), left superior temporal gyrus (STG)) for volume-of-interest analyses.

Compared to SSD-CI with good performance, those with poor performance showed poorer word understanding during unilateral stimulation of the NH ear. The activation of the right PAC was significant and comparable across groups. The asymmetry index of rCBF of the PAC decreased significantly in NH controls and SSD-CI with good performance from uni- to bilateral stimulation, but not in SSD-CI with poor performance. A significant activation of left STG was only observed in NH controls and SSD-CI with good performance.

This study underlines the usefulness of [15O]water PET to explore regional neuronal mechanisms underlying CI performance: SSD-CI with poor performance are characterized by a low normal word understanding with the NH ear only (in line with the lack of STG activation) and compromised binaural integration, although the CI provides quantitatively comparable input to the contralateral PAC in both SSD-CI groups.

The online version contains supplementary material available at 10.1007/s00259-025-07639-8.