Bilateral Speech-Evoked Envelope Following Responses: Benefit and Impact of Interaural Asymmetries

OBJECTIVES: Speech-evoked envelope following responses (EFRs) are a promising objective metric to assess aided access to speech. However, most studies have used unilateral stimulation and therefore do not reflect typical everyday listening, which is usually bilateral. The purpose of this study was to investigate: (i) the benefits of bilateral over unilateral stimulation in terms of increased EFR amplitudes, and (ii) the influence of clinically relevant interaural asymmetries, including relatively large interaural time and level differences (ITDs, ILDs) potentially caused by unilateral hearing aid fitting and asymmetry in hearing thresholds, respectively. DESIGN: In 42 adults with normal hearing, EFRs were elicited by male-spoken tokens /sashi/ or /sa/. The tokens enabled eliciting multiple EFRs, one at the fundamental frequency (f0) of voice (f0 EFRs; typically at ~80 to 100 Hz) by low, mid and high frequency phoneme bands, and one by the syllabic structure (slow-rate EFRs;