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

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.

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; <8 Hz). To evaluate the benefit of bilateral over unilateral stimulation, stimuli were presented bilaterally and unilaterally at 65, 50, 35, and 15 dB SPL. To evaluate the effect of ILDs, the stimulus in the right ear was held constant at 65 dB SPL whilst the stimulus in the left ear was lowered to create ILDs of 15, 30, and 50 dB. To evaluate the effect of ITD, the stimulus was delayed by 5.6 msec in the left relative to the right ear to simulate a left unilateral hearing aid fitting. EFRs were measured with a non-inverting electrode at the vertex and an inverting electrode at the nape.

Between 35 and 65 dB SPL, bilateral f0 EFR amplitudes were 80 to 127% larger than their unilateral counterparts. Relatively smaller (20 to 46%) increases in amplitude were evident for the slow-rate EFRs. Detection rates improved mainly at lower levels, increasing by up to 50% and 15% in bilateral relative to unilateral conditions for f0 and slow-rate EFRs, respectively. The large 5.6-msec ITD attenuated f0 EFR amplitudes significantly, but had no significant effect on the slow-rate EFRs. ILD reduced f0 and slow-rate EFR amplitudes, but had no noticeable impact on detection rates. When stimuli were audible in both ears (ILD < 30 dB), the bilateral f0 and slow-rate EFR amplitudes were slightly smaller than the sum of their unilateral counterparts, suggesting the presence of binaural interactions. The ratio of the bilateral to the unilateral sum of response amplitudes was 88 to 93% and 62 to 73% for the f0 and slow-rate EFRs, respectively.

The significant bilateral advantage in f0 and slow-rate EFR amplitudes, especially for stimuli closer to hearing thresholds, encourages the use of bilateral stimulation where appropriate. While a large 5.6 msec ITD-such as those introduced by a unilateral hearing aid-reduced f0 EFR amplitudes, slow-rate EFRs remained unaffected. Thus, unilateral hearing aid fittings in sound field might leverage slow-rate EFRs to prevent false negatives. Bilateral advantages were preserved despite ILDs, provided the better ear had sufficient audibility. This study provides the first evidence of observable binaural interactions in 80 to 100 Hz EFRs.