PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Intelligibility of filtered speech and its relation toelectrophysiological markers of supra-threshold hearing deficits
Markus GARRETT1; Viacheslav VASILKOV2; Sarah VERHULST2
1Medizinische Physik and Cluster of Excellence Hearing4all, Oldenburg University, Germany

2

Hearing Technology @ WAVES, Dept of Information Technology, Ghent University, Belgium

ABSTRACTThe origins and perceptual consequences of supra-threshold hearing deficits are not wellunderstood. Sensory and neural deficits are likely contributors, underlining the need for an objective
measure which quantifies the neural contributors and complements audiometric measures of sensory
loss. The envelope-following response (EFR) is sensitive to neural fiber loss in animal models and has
been proposed as a marker. However, its diagnostic sensitivity in humans and relationship to speech
intelligibility are still unclear. Here we argue that the relationship between electrophysiologicalmeasures of temporal coding ability and the performance in speech-in-noise tasks crucially depends on
the underlying stimulus parameters. We discuss ways in which experimental protocols can be adjusted
to improve the predictive relationship between electrophysiological measures of temporal coding
fidelity and behavioral measures of speech intelligibility and test these predictions in a group of young
normal-hearing, elderly normal-hearing and elderly hearing-impaired participants as groups reflecting
different degrees of neural or sensory hearing deficits.
Keywords: envelope following response, supra-threshold hearing deficits, speech intelligibility, filteredspeech

1. INTRODUCTION
An important part of the characterization of a person's hearing status is a quantification of theirability to understand speech in the presence of background noise. Even though speech audiometry innoise gives an accurate representation of one's ability to understand speech, it does not provide
information about the underlying deficits at play. Additional threshold measures such as theaudiometric threshold detection and otoacoustic emission measures are routinely used to quantify
deficits linked to outer-hair-cell loss. However, as speech is usually presented at supra-thresholdhearing levels, these threshold metrics are not informative about a person's supra-threshold temporal
coding abilities (1), which are crucial to extract different features from the speech input to ultimately
make sense of it.
Animal models have shown that electrophysiological metrics such as the envelope following
response (EFR) and the auditory brain stem response (ABR) are able to capture facets of temporal
coding deficits. The EFR (e.g. 2) measures the phase-locked neural response of the brain to thestimulus envelope and shows, depending on the stimulus material, characteristic peaks coinciding withthe modulation frequency and its harmonics in the frequency-domain response of an EEG recording
using a standard electrode configuration (e.g. vertex vs. ear-lobe reference). The ABR is atransient-evoked response (3,4) elicited by a click or tone burst. However, the translation of theEFR/ABR findings to humans has proven difficult due to species-dependent differences and large
inter-individual differences in human cohorts. The available literature has so far shown mixed results
regarding the applicability of these electrophysiological measures for the objective quantification ofsupra-threshold hearing deficits in humans (5).
Especially the EFR, as a measure of temporal coding ability in the early auditory pathway, should
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markus.garrett@uol.deviacheslav.vasilkov@ugent.be; sarah.verhulst@ugent.be5647

present a strong relation to the ability to understand speech when presented in difficult listening
environments (6). Nevertheless, many studies that have investigated the relationship between the EFRor ABR with measures of speech intelligibility did not show any consistent relationships (e.g. 7–9). It
was therefore often concluded that these EEG measure offer limited sensitivity to supra-thresholdhearing deficits (see (10) for a review on the topic) even though there are examples where empiricalfindings were consistent with theoretical considerations (e.g. 1,11,12).
Here we argue that the relationship between electrophysiological measures of temporal coding
ability and the performance in speech-in-noise tasks crucially depends on the stimulus characteristics.
We discuss ways in which experimental protocols can be adapted to improve the relationship between
electrophysiological measures of temporal coding fidelity and behavioral measures of speech
intelligibility.

2. RESULTS & DISCUSSION
The ability to make sense of speech, crucially relies on the extraction of speech features such as thefast fluctuations in the fine structure (TFS) or the much lower and globally varying envelope of thespeech signal (ENV). As fine structure information are predominantly extracted from low frequency
regions (< 1 kHz, (13)) and envelope coding takes place at higher frequency regions (14), the dominantfeatures used for speech extraction thereby seem to strongly depend on the available frequency content
of the speech signal.
As temporal coding abilities degrade with advanced age and/or exposure to noise, the relative
contributions of different features of speech facilitating speech intelligibility might therefore also vary
between young normal-hearing, older normal-hearing and clinical populations with sensorineural
hearing loss. In this context it is also important to consider that hearing loss classica lly affects thebasal part of the cochlear, predominantly processing the high frequency content of the speech.
Most stimulus paradigms for speech intelligibility assessment use broadband speech material torepresent natural speech encountered in everyday life. On the other hand, the corresponding stimulus
material used to evoke the EFR mostly consist of either a pure tone carrier at frequencies around 4 kHz
or a broadband white noise carrier. These carriers are afterwards amplitude-modulated using
modulation frequencies in the 80-120 Hz range and presented to the subject over earphones. The
individual strength of the EFR peak is an objective measure of temporal coding and is typicallycompared to a speech intelligibility metric (e.g., speech reception thres hold, SRT) to understand thecontribution of temporal coding deficits to speech intelligibility. The interpretation of the EFR vs. SRT
comparison constitutes the following problems: i) temporal coding ability of a narrow portion of thecochlea (in the case of a pure-tone EFR) is compared with the intelligibility of a broadband speechstimulus, ii) the envelope coding performance of the whole cochlea (in the case of a broadband EFR)
is compared to the intelligibility of broadband speech which might depend on many different featuresof speech (e.g., TFS) and not only the envelope information as measured by the EFR. These
comparisons therefore either lead to a mismatch of frequency content and/or mismatch of underlying
mechanism at play. We therefore argue that these comparisons likely compare 'apples and pears' andthat it is not surprising that measures do not show any kind of consistent relationship.
To overcome these mismatches and to strengthen the relationship between these metrics, wesuggest the use of a frequency-specific carrier signal within a frequency region know to use envelopeinformation (e.g. 4 kHz) and compare these to the speech intelligibility performance of a filteredspeech signal (e.g. > 1.6 kHz) (15). We argue that the underlying mechanisms of the metrics computedin the afore mentioned way will be comparable and therefore yield much stronger relationships
compared to classically employed stimulus paradigms.
To explore the variables in the relationship between EFRs and speech intelligibility, we conducteda study which investigated the match and mismatch between EFR and speech intelligibility measuresin a cohort of young normal-hearing, elderly normal-hearing and elderly hearing-impaired participants.
Our results confirm that the relationship between both metrics grows stronger as the frequency content
of both stimuli match the regions of suspected envelope coding, showing the highest correlations forhigh-pass filtered speech with a 120 Hz amplitude-modulated 4 kHz pure tone carrier.

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3. CONCLUSIONS
Our results support the underlying assumptions that the mixed results found in literature between
objective supra-threshold electrophysiological measures of temporal coding and the behavioralmeasures of speech-in-noise tasks (e.g. 7–9) are likely due to the mismatch of underlying frequency
content and/or coding mechanisms at play. We provide evidence that the manipulation of both speechand AM-stimuli towards better matching the frequency content and coding mechanism will yieldsignificant relationships between the metrics in agreement with theoretical considerations. A detailed
account of the results of our work will be presented in our talk at the ICA 2019.

ACKNOWLEDGEMENTS
The authors disclose receipt of the following financial support for the research, authorship, and/or
publication of this article: This work was supported by the German Research Foundation (DFG Clusterof Excellence EXC 1077/1 "Hearing4all": MG, SV; PP1608: VV) and the European Research Council(ERC) under the Horizon 2020 Research and Innovation Programme (grant agreement No 678120
RobSpear, VV, SV).”

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