The syllable in the light of motor skills and neural oscillations

Abstract

International audienceRecent advances in neuroscience have brought a great focus on how the auditory cortex tracks speech at certain time scales corresponding to pre-lexical speech units in order to achieve comprehension. In particular, it has been claimed that it is the syllabic rhythm to which slow neural oscillations in the auditory cortex entrain in order to chunk the speech stream into smaller informational units. However, the terms “syllable” and “rhythm” have been treated quite loosely in the current literature. We revisit classic approaches to show that both concepts do not necessarily have an acoustic or phonetic counterpart, which could be directly extracted by neural processes. We would like to suggest that the syllabic rhythm could emerge at the intersection of acoustic–phonetic and motor knowledge of speech. We furthermore propose that nesting of cortical oscillations might be the key mechanism to understand the timing constraints that lead to the emergence of the syllable

Perceptually relevant speech tracking in auditory and motor cortex reflects distinct linguistic features

“…These phrases had distinct pauses (see Fig 1 for an example sentence) that determined the rhythm of the sentence (also visible in the frequency spectrum S3A Fig ). The syllable rate is generally difficult to assess [ 18 , 79 ]. Here, we chose to count the actually produced syllables for each sentence.…”

Characterizing speech rhythm using spectral coherence between jaw displacement and speech temporal envelope

“…The mngu0 1 contains one male English speaker producing over 1,000 utterances, amongst which 594 in the duration range of [2,8] sec were chosen for the present study. The 2sec cutoff allowed at least one cycle of the lowest δ frequency (0.5 Hz) to be included; the 8-sec cutoff excluded sentences with medial pauses.…”

“…Rhythmic frames play a vital role in the neurological processing of speech: the recurring oscillations in the ENV facilitate the brain to parse the incoming speech signal for comprehension. It has been demonstrated that the δoscillation (0.5-3 Hz, corresponding to stress/foot rates) and θ-oscillation (3-9 Hz, corresponding to syllable rates) in the auditory cortex entrain to the speech ENV at these modulation rates [6][7][8][9] . These slow neuronal oscillations formulated a temporal window structure whereby the auditory cortex tracks the speech signal at the foot and syllable rates.…”

“…The jaw oscillation and speech ENV possess strong spectral coherence in the frequency bands pertaining to the lower neuronal oscillations (δ-and θ-rates), upholding the joint role of jaw oscillation and speech ENV in creating the rhythmic frames. The jaw produces lower modulation frequencies in the speech signal which the auditory cortex of to[6][7][8][9] ; the jaw movements also invite neuronal entrainment in the listener's visual cortex 10 . These entrainments play a vital role in speech processing and comprehension.…”

Brain oscillations in language comprehension

“…Acoustic targets are more accurately detected when they occur within a chunking pattern, especially when the chunking rate is within the frequency range of neuronal delta. Strauß and Schwartz (2017) focus on syllabic speech processing, and challenge the traditional view on syllabic rhythms, which in their view are not clearly defined in terms of acoustic of phonetic features. They suggest that the emergence of syllables as sub-units of speech is constrained by nested cortical oscillations, and reflects the interplay of acoustic-phonetic and motor representations.…”