Effect of Stimulus Level and Bandwidth on Speech-Evoked Envelope Following Responses in Adults With Normal Hearing

Easwar, Vijayalakshmi1; Purcell, David W.1,2; Aiken, Steven J.3; Parsa, Vijay1,2; Scollie, Susan D.1,2

doi: 10.1097/AUD.0000000000000188
Research Articles

Objective: The use of auditory evoked potentials as an objective outcome measure in infants fitted with hearing aids has gained interest in recent years. This article proposes a test paradigm using speech-evoked envelope following responses (EFRs) for use as an objective-aided outcome measure. The method uses a running speech-like, naturally spoken stimulus token /susa∫i/ (fundamental frequency [f0] = 98 Hz; duration 2.05 sec), to elicit EFRs by eight carriers representing low, mid, and high frequencies. Each vowel elicited two EFRs simultaneously, one from the region of formant one (F1) and one from the higher formants region (F2+). The simultaneous recording of two EFRs was enabled by lowering f0 in the region of F1 alone. Fricatives were amplitude modulated to enable recording of EFRs from high-frequency spectral regions. The present study aimed to evaluate the effect of level and bandwidth on speech-evoked EFRs in adults with normal hearing. As well, the study aimed to test convergent validity of the EFR paradigm by comparing it with changes in behavioral tasks due to bandwidth.

Design: Single-channel electroencephalogram was recorded from the vertex to the nape of the neck over 300 sweeps in two polarities from 20 young adults with normal hearing. To evaluate the effects of level in experiment I, EFRs were recorded at test levels of 50 and 65 dB SPL. To evaluate the effects of bandwidth in experiment II, EFRs were elicited by /susa∫i/ low-pass filtered at 1, 2, and 4 kHz, presented at 65 dB SPL. The 65 dB SPL condition from experiment I represented the full bandwidth condition. EFRs were averaged across the two polarities and estimated using a Fourier analyzer. An F test was used to determine whether an EFR was detected. Speech discrimination using the University of Western Ontario Distinctive Feature Differences test and sound quality rating using the Multiple Stimulus Hidden Reference and Anchors paradigm were measured in identical bandwidth conditions.

Results: In experiment I, the increase in level resulted in a significant increase in response amplitudes for all eight carriers (mean increase of 14 to 50 nV) and the number of detections (mean increase of 1.4 detections). In experiment II, an increase in bandwidth resulted in a significant increase in the number of EFRs detected until the low-pass filtered 4 kHz condition and carrier-specific changes in response amplitude until the full bandwidth condition. Scores in both behavioral tasks increased with bandwidth up to the full bandwidth condition. The number of detections and composite amplitude (sum of all eight EFR amplitudes) significantly correlated with changes in behavioral test scores.

Conclusions: Results suggest that the EFR paradigm is sensitive to changes in level and audible bandwidth. This may be a useful tool as an objective-aided outcome measure considering its running speech-like stimulus, representation of spectral regions important for speech understanding, level and bandwidth sensitivity, and clinically feasible test times. This paradigm requires further validation in individuals with hearing loss, with and without hearing aids.

A test paradigm based on speech-evoked envelope following responses (EFRs) is proposed as an objective hearing aid outcome measure. A male naturally-spoken token /susa∫i/ was modified to elicit individual EFRs by eight carriers representing low, mid and high frequency spectral regions. The study evaluated the effects of stimulus level and bandwidth in normal hearing adults. Results demonstrated carrier-specific sensitivity to level and bandwidth, as well as convergent validity when compared with bandwidth-related changes in behavioral measures such as speech discrimination and sound quality rating. The current sample also shows clinically feasible test times.

1National Centre for Audiology, Western University, London, Ontario, Canada; 2School of Communication Sciences and Disorders, Western University, London, Ontario, Canada; and 3School of Human Communication Disorders, Dalhousie University, Halifax, Nova Scotia, Canada.

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This study was funded by Western Graduate Research Scholarship, Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, the Ontario Ministry of Research and Innovation-Early Researcher Awards and the Ontario Research Fund-Research Excellence award.

Received September 23, 2014; accepted May 3, 2015.

Address for correspondence: Vijayalakshmi Easwar, National Centre for Audiology, 2262 Elborn College, Western University, London, Canada N6G 1H1. E-mail: veaswar@nca.uwo.ca

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