Neural speech tracking shifts from the syllabic to the modulation rate of speech as intelligibility decreases

Abstract

The most prominent acoustic features in speech are intensity modulations, represented by the amplitude envelope of speech. Synchronization of neural activity with these modulations is vital for speech comprehension. As the acoustic modulation of speech is related to the production of syllables, investigations of neural speech tracking rarely distinguish between lower-level acoustic (envelope modulation) and higher-level linguistic (syllable rate) information. Here we manipulated speech intelligibility using noise-vocoded speech and investigated the spectral dynamics of neural speech processing, across two studies at cortical and subcortical levels of the auditory hierarchy, using magnetoencephalography. Overall, cortical regions mostly track the syllable rate, whereas subcortical regions track the acoustic envelope. Furthermore, with less intelligible speech, tracking of the modulation rate becomes more dominant. Our study highlights the importance of distinguishing between envelope modulation and syllable rate and provides novel possibilities to better understand differences between auditory processing and speech/language processing disorders.

The channel capacity of multilevel linguistic features constrains speech comprehension

“…These values parallel theoretical considerations and neurophysiological observations (Giraud & Poeppel, 2012;Giroud et al, 2020) and provide a behavioral validation that phonemic sampling occurs at such a rate (see also (Marchesotti et al, 2020). While the acoustic modulation and syllabic rates are often reduced to one another, they are dissociable (see also (Schmidt et al, 2021), are associated with different processing bottlenecks, but both unfold at around 5 Hz in natural speech and have a channel capacity of around 15 Hz. This result strongly suggests that both low-level acoustic and language-specific rhythmic processes contribute to speech comprehension.…”

Speech intelligibility changes the temporal evolution of neural speech tracking

“…The envelope modulation rate of the 24 audio files in the original, 7-ch, 5-ch, 3-ch, 2-ch, and 1-ch vocoded condition was 5.61 Hz, 5.53 Hz, 5.27 Hz, 5.23 Hz, 4.93 Hz, and 4.83 Hz, respectively. A main effect of vocoding was observed [ 𝒳 2 (5) = 206.0, p = 1.83 × 10 −42 , Kendall’s W = 0.134, Friedman test], in line with Schmidt et al (2021).…”

“…Late responses around 315-380 ms (M350 TRF ) were strongest at medium vocoding levels as compared to clear and less intelligible speech. To complement our findings in TRF, we also computed speech envelope reconstruction, speech-brain coherence, and periodic components (peak center frequency, peak bandwidth, peak height) and aperiodic components (exponent, offset) of broadband coherence spectra (Schmidt et al, 2021). Relating the TRF components to those neural measures, we found that M50 TRF and M200 TRF were correlated with the center frequency of the broadband coherence spectra in a different direction.…”

Individual prediction tendencies facilitate cortical speech tracking

“…Our findings show, for the first time, that the tendency to anticipate low-level acoustic features according to their contextual probability facilitates the cortical tracking of the speech envelope. Tracking of information conveyed by the speech envelope is critical for speech comprehension and the processing of low-level linguistic features (Schmidt et al, 2021; Vanthornhout et al, 2018). This suggests a potentially supportive role of the tendency to anticipate low-level acoustic features on the processing of naturally spoken language.…”