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 supports speech comprehension. As the acoustic modulation of speech is related to the production of syllables, investigations of neural speech tracking commonly do not 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.…”

Early language experience modulates the cortical tracking of speech

“…We propose that envelope-level CTS could thus serve as a route for speech comprehension during language learning, possibly by compensating while increasing higher-order linguistic knowledge is being learned. In line with this interpretation, studies show that adults rely more on envelopelevel CTS under noisy and effortful conditions than under normal conditions (Hauswald et al, 2022;Schmidt et al, 2023). Thus, it is likely that as individuals can exploit an increasingly proficient language model for CTS, they gradually shift from relying on low-level temporal information towards tracking higher order linguistic units and structures.…”

“…It is relevant to note that, while tracking speech AMs contributes to the development of phonological skills (Leong & Goswami, 2014, 2017Molinaro et al, 2016), studies in adults highlight that CTS might be more reliant on speech rate than on speech envelope modulations, at least in normal and effortless listening environments (Hauswald et al, 2022;Schmidt et al, 2023). Therefore, the role of acoustic-temporal CTS for language development might decrease as soon as individuals can rely on sufficient lexico-semantic knowledge to track continuous speech (Kaufeld et al, 2020;Molinaro et al, 2021).…”

Predicting the Beat Bin: Beta Oscillations Predict the Envelope Sharpness in a Rhythmic Sequence

“…We investigated neural processes underlying the prediction of the temporal precision of auditory events during beat perception. Flexibility in perceptual temporal precision allows us to perceive regularity in acoustic information that does not show a strictly periodic pattern or has a shape that makes its perceived temporal position indistinct, as can be the case in language (16,17,19,22) or music (6,14). Although neural entrainment is often associated with strictly isochronous stimuli, the underlying neural mechanisms seem to tolerate a certain amount of temporal variability (5).…”

“…Accordingly, we will use the term precision when referring to the width of a beat bin throughout the rest of the paper. In the same vein, it has been shown for speech processing, that even if the acoustic sequence is not perfectly isochronous, neural entrainment is tolerant to timing variability ( (5,(16)(17)(18) but see (19)). In perceptual neuroscience, a systematic investigation of the effect of the auditory stimulus shape on neural entrainment is missing (but see (20,21)), except for studies investigating neural tracking of the acoustic envelope during speech perception (22).…”