Cortical mechanisms of across-ear speech integration investigated using functional near-infrared spectroscopy (fNIRS)

TL;DR

This study explores how the brain integrates speech heard in both ears, using a special imaging technique to compare normal hearing and simulated hearing loss conditions.

Contribution

The study introduces a novel use of fNIRS to investigate cortical mechanisms of across-ear speech integration under simulated cochlear implant conditions.

Findings

The V-shaped speech intelligibility function was only observed in the bilateral cochlear implant condition.

Cortical activity in auditory and prefrontal regions varied significantly with alternating speech rates in normal hearing but not in simulated implant conditions.

Degraded speech in one or both ears altered integration strategies, as reflected in distinct cortical activity patterns.

Abstract

This study aimed to investigate integration of alternating speech, a stimulus which classically produces a V-shaped speech intelligibility function with minimum at 2–6 Hz in typical-hearing (TH) listeners. We further studied how degraded speech impacts intelligibility across alternating rates (2, 4, 8, and 32 Hz) using vocoded speech, either in the right ear or bilaterally, to simulate single-sided deafness with a cochlear implant (SSD-CI) and bilateral CIs (BiCI), respectively. To assess potential cortical signatures of across-ear integration, we recorded activity in the bilateral auditory cortices (AC) and dorsolateral prefrontal cortices (DLPFC) during the task using functional near-infrared spectroscopy (fNIRS). For speech intelligibility, the V-shaped function was reproduced only in the BiCI condition; TH (with ceiling scores) and SSD-CI conditions had significantly higher scores across all alternating rates compared to the BiCI condition. For fNIRS, the AC and DLPFC exhibited significantly different activity across alternating rates in the TH condition, with altered activity patterns in both regions in the SSD-CI and BiCI conditions. Our results suggest that degraded speech inputs in one or both ears impact across-ear integration and that different listening strategies were employed for speech integration manifested as differences in cortical activity across conditions.

Figures