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Neural entrainment to continuous speech is typically observed within the language network and can be modulated by both low-level acoustic features and high-level meaningful linguistic units (e.g., phonemes, phrases, and sentences). Recent evidence showed that visual cortex may entrain to speech envelope, however its putative role in the hierarchy of speech processing remains unknown. We tested blindfolded participants who listened to semantically meaningful or meaningless stories, either in quiet or embedded in multi-talker babble noise. Entrainment to speech was assessed with forward linear modeling of participants' EEG activity. We investigated (1) low-level acoustic effects by contrasting neural tracking of speech presented in quiet or noise and (2) high-level linguistic effects by contrasting neural tracking to meaningful or meaningless stories. Results showed that envelope tracking was enhanced and delayed for speech embedded in noise compared to quiet. When semantic information was missing, entrainment to speech envelope was fastened and reduced. Source modeling revealed that envelope tracking engaged wide neural networks beyond the auditory cortex, including early visual cortex. Surprisingly, while no clear influence of semantic content was found, the magnitude of visual cortex entrainment was affected by low-level features. The decrease of sound SNR-level dampened visual cortex tracking, suggesting an active suppressing mechanism in challenging listening conditions. Altogether, these findings provide further evidence of a functional role of early visual cortex in the entrainment to continuous speech.

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The auditory system relies on local and global representations, depending on the amount of entering temporal details. When local information exceeds system constraints, acoustic details are summarized into a set of average statistics and global structure emerges. Whether computations subtending local and global representations of sounds can be distinguished at the cortical level is unknown. A computational auditory model was employed to extract auditory statistics from natural sound textures, such as fire, wind, and rain, and generate synthetic exemplars in which we had control of local and global properties. Participants were passively exposed to auditory streams with triplets of sounds including two standards and an oddball while the EEG was recorded. In Local Discrimination (exp 1) the oddball sound varied from standards only for its local features; in Global Discrimination (exp 2), it varied for its global structure. Different sound durations were employed to manipulate the amount of acoustic information and statistical variability between standards and oddballs. We expected short and long sounds to engage local (exp 1) or global (exp 2) representations, respectively. As predicted, neural responses associated with local discriminations exceeded global ones for short sounds, whereas the opposite pattern emerged for longer sounds. Electrical source estimates revealed that local discriminations mainly engaged auditory regions of the right hemisphere, while global discriminations of the left hemisphere. These findings revealed distinct neural correlates of acoustic discriminations guided by local or global structures. Ultimately results demonstrated that the auditory system developed a functional architecture to automatically detect changes occurring in the environment based on these two modes of sound representation.SIGNIFICANT STATEMENTPrior to this study, it was not known whether it was possible to directly measure auditory cortical representations of sound discriminations depending on local or global acoustic properties. Results unveiled the existence of a mechanisms which pre-attentively determines at which timescale (local or global) a change in a stream occurred based on statistical variability between sound excerpts. By performing source reconstruction, we additionally revealed that the response to local change detection pertains to different cortical structures as compared to global one, displaying also hemispheric asymmetries. These findings expand our knowledge on fundamental mechanisms characterizing auditory system functioning.