其他摘要 | Language is an essential tool for human communication. Research in the past decades has shown that the production and perception/comprehension of speech information may share the same neural circuits. This is the sensorimotor integration in speech processing, namely "the listening in speaking, and the speaking in listening". Speech perception and comprehension have been found to engage the dorsal articulatory motor cortical pathways located in the left frontal and parietal lobes. Particularly, this involvement offers compensation to the ventral pathways for auditory encoding and lexical retrieval located in the temporal cortices. On the other hand, the left motor cortex is engaged in speech perception with effector-specific somatotopy as in speech production. However, the sensorimotor integration mechanisms in speech perception at relatively micro and macro spatiotemporal scales remain unclear. At the micro-spatial scale, whether and how the laryngeal motor cortex (LMC) is recruited in the perceptual decision of the speech cues that it controls in articulation is unknown. At the micro-temporal scale, it remains debatable regarding the particular stages of motor involvement in the speech perceptual decision. At the macro一spatial scale, how bilateral auditory and motor cortices are cooperatively engaged in speech perception is incompletely characterized. At the macro-temporal scale, how the motor cortices subserve the perception and comprehension of hierarchically organized continuous speech is elusive. This dissertation reports three studies that explored these topics from multiple perspectives concentrating on the perception and comprehension of Chinese, a tonal language. Studies included two systematic reviews of previous research and four experiments and integrated meta-analysis, neuroimaging, and non-invasive brain stimulation techniques as well as computational modeling of multi-modal behavioral and neuro-electrophysiological data.
Study 1 includes two systematic reviews (Analysis 1 for a meta-analysis of neuroimaging studies on the perception of speech cues; Analysis 2 for a target-based systematic review of non-invasive brain stimulation studies on speech perception and comprehension. Analysis 1 found that the auditory and motor cortices are both engaged in the perceptual decision of speech features, and the spatial distributions of brain activations for the perception of different cues (segmental phoneme, lexical tone, and prosody) reflect the acoustic properties, linguistic functions, as well as articulatory motor gestures in terms of the gradient of representation and hemispheric functional asymmetry. Analysis 2 found more causal evidence for the recruitment of the left motor cortex in the perception of lexical phonemic cues, and that of the right motor cortex in the perception of non-lexical prosodic features. Moreover, in speech perception, the dorsal stream that contains the motor cortices cooperates with the ventral stream through multiple functional connectivity pathways.
Study 2 investigated the brain mechanisms subserving sensorimotor integration of the perception of short phonemes and syllables. Study 2 used transcranial magnetic stimulation (TMS) to interfere with the left or right LMC and measured the effects on the perceptual decision of Mandarin lexical tone and consonant, through psychometric curve fitting as well as drift-diffusion modeling (DDM) of reaction time distributions. Experiment 1 utilized repetitive TMS (rTMS), whereas Experiment 2 applied theta- burst stimulation (TBS), and Mandarin-speaking participants performed similar tasks in the two experiments. Psychometric curve fitting in the two experiments convergently showed that bilateral LMCs are involved in the perceptual decision of speech features in an effector-specific manner. Moreover, curve-fitting results in Experiment 2 showed that the left LMC is relatively essential in the perceptual decision of lexical tone and consonant, while its right counterpart is causally engaged in difficult tasks. Meanwhile, the DDM in Experiment 2 demonstrated that the activations of the LMC contribute to various stages of speech perceptual decision as a function of the hemisphere and task difficulty.
Study 3 used transcranial alternating current stimulation (tACS, Experiment 3), electroencephalography (EEG, Experiment 3), and magnetoencephalography (MEG, Experiment 4) techniques to investigate the causal engagement of the sensorimotor circuit in sentence perception (Experiment 3) and the sensorimotor integration neural mechanisms of sentence and discourse comprehension (Experiment 4). Experiment 3 applied in-phase or anti-phase 4-Hz dual-site tACS upon participants' auditory and motor cortices when they performed sentence repetition tasks in noise in order to detect changes in repetition scores after the auditory-motor low-frequency neural oscillation coupling being enhanced or hampered, correspondingly. Experiment 3 found that antiphase tACS impaired the performance in sentence repetition in speech spectrum noise. Meanwhile, the speech-tracking accuracies of the low-frequency EEG phase and alphaband EEG power in the same tasks without tACS significantly predicted stimulation effects. To investigate the neural tracking of multi-dimensional speech acoustic features in the auditory and motor cortices, Experiment 4 recorded MEG signals when participants listened to a story read in Mandarin and used the temporal response function (TRF) to quantify the encoding capacities of the temporal envelope, pitch contour, and voicing onset/offset at the sensor level. Experiment 4 found that lowfrequency (delta and alpha bands) neural oscillations subserve the encoding of all three features, and the alpha band oscillations also support envelope encoding. In the "cocktail party" situation (masked by speech), the left motor cortex keeps a certain encoding ability, and this occurs at the low frequency and alpha band for envelope encoding, and at the low-frequency bands for pitch contour and voicing onset/offset encoding. Two experiments from Study 3 convergently showed that the left motor cortex compensates for the encoding of speech acoustic features in challenging tasks, and the cross-frequency coupling of neural oscillations between the frontoparietal and the temporal areas may support this compensation.
In summary, this dissertation reports that, at the micro一spatial scale, the LMC is involved in speech perception with effector-specificity that constitutes a "motor strip" along with the lip and tongue motor areas. At the micro-temporal scale, the motor cortex is engaged in various phases of speech perceptual decision. At the macro-spatial scale, the hemispheric functional asymmetry of bilateral motor cortices is related to their roles in the articulation and the linguistic functions of the speech cues and is left-dominant. At the macro-temporal scale, the motor cortex encodes multi一dimensional speech cues and assists auditory encoding at adverse listening conditions. The low-frequency (delta and theta bands) auditory-motor neural oscillation coupling and the alpha band signals may subserve this integration. This dissertation deepens the knowledge of the brain mechanisms and theoretical models of sensorimotor integration in Chinese speech perception and is potentially constructive for clinical interventions on speech disabilities and the development of brain-inspired artificial intelligence. |
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