The effect of stimulus intensity on neural envelope tracking

Abstract

Objectives: The last years there has been significant interest in attempting to recover the temporal envelope of a speech signal from the neural response to investigate neural speech processing. The research focus is now broadening from neural speech processing in normal-hearing listeners towards hearing-impaired listeners. When testing hearing-impaired listeners speech has to be amplified to resemble the effect of a hearing aid and compensate peripheral hearing loss. Until today, it is not known with certainty how or if neural speech tracking is influenced by sound amplification. As these higher intensities could influence the outcome, we investigated the influence of stimulus intensity on neural speech tracking. Design: We recorded the electroencephalogram (EEG) of 20 normal-hearing participants while they listened to a narrated story. The story was presented at intensities from 10 to 80~dB~A. To investigate the brain responses, we analyzed neural tracking of the speech envelope by reconstructing the envelope from EEG using a linear decoder and by correlating the reconstructed with the actual envelope. We investigated the delta (0.5-4 Hz) and the theta (4-8 Hz) band for each intensity. We also investigated the latencies and amplitudes of the responses in more detail using temporal response functions which are the estimated linear response functions between the stimulus envelope and the EEG. Results: Neural envelope tracking is dependent on stimulus intensity in both the TRF and envelope reconstruction analysis. However, provided that the decoder is applied on data of the same stimulus intensity as it was trained on, envelope reconstruction is robust to stimulus intensity. In addition, neural envelope tracking in the delta (but not theta) band seems to relate to speech intelligibility. Similar to the linear decoder analysis, TRF amplitudes and latencies are dependent on stimulus intensity: The amplitude of peak 1 (30-50 ms) increases and the latency of peak 2 (140-160 ms) decreases with increasing stimulus intensity. Conclusion: Although brain responses are influenced by stimulus intensity, neural envelope tracking is robust to stimulus intensity when using the same intensity to test and train the decoder. Therefore we can assume that intensity is not a confound when testing hearing-impaired participants with amplified speech using the linear decoder approach. In addition, neural envelope tracking in the delta band appears to be correlated with speech intelligibility, showing the potential of neural envelope tracking as an objective measure of speech intelligibility.

Hearing loss is associated with delayed neural responses to continuous speech

“…In several studies, it has been shown that increasing task demand due to lower stimulus intensity or increasing background noise is associated with an increase in the latency of neural responses in continuous speech (Mirkovic et al, 2019;Verschueren et al, 2020) or in CAEP-responses (Billings et al, 2015;Van Dun et al, 2016;Maamor and Billings, 2017;McClannahan et al, 2019). Similarly, for NH listeners, we found that as speech understanding decreases, the delay of neural responses increases.…”

“…McClannahan et al (2019) remarked that differences in the audibility of the stimulus might explain the differences in neural response latency. Indeed, Verschueren et al (2020) observed that reduced audibility increases the latency of the neural responses to continuous speech. However, at a comfortable loudness (at intensities of 60 dB or higher in a NH population), the latency reaches a plateau.…”

“…In several studies, it has been shown that NH listeners show an increased neural response latency with increasing task demand due to lower stimulus intensity, increasing background noise or stimulus vocoding. This is the case for neural processing of continuous speech (Mirkovic et al, 2019; Verschueren et al, 2020; Kraus et al, 2020) as well as simple sounds (Billings et al, 2015; Van Dun et al, 2016; Maamor and Billings, 2017; McClannahan et al, 2019). Our results show that this increase in latency is absent for adults with a higher degree of hearing loss (Figure 5.C).…”

“…Recently, Verschueren et al (2020) observed that reduced audibility increases the latency of the neural responses to continuous speech. However, at a comfortable loudness (at intensities of 60 dB or higher in a NH population), the latency reaches a plateau.…”