Research study: "Cortical tracking of speech is reduced in adults who stutter when listening for speaking" (2024)

Research study: "Cortical tracking of speech is reduced in adults who stutter when listening for speaking" (2024) Research: [https://www.biorxiv.org/content/10.1101/2024.02.23.581767v1](https://www.biorxiv.org/content/10.1101/2024.02.23.581767v1) Full PDF: [https://www.biorxiv.org/content/10.1101/2024.02.23.581767v1.full.pdf](https://www.biorxiv.org/content/10.1101/2024.02.23.581767v1.full.pdf) ## Abstract This study explores cortical tracking of speech (CTS) in adults who stutter (AWS) compared to typically fluent adults (TFA) while listening to sentences. We manipulated the upcoming involvement of the speech-motor system during listening: participants either had to simply listen to the sentences (listening only) or complete unfinished sentences by naming a picture (listening-for-speaking). AWS, known for atypical neural structure and functionaing in the speech-motor network, exhibited reduced CTS in the theta band in temporal sensors during the listening-for-speaking task, reflected at the source level in the left temporo-parietal junction and the right pre-motor and supplementary motor regions. Additionally, connectivity analyses reveal that TFA had stronger inter- and intra-hemispheric information transfer in the theta range than AWS in both tasks, involving frontal, temporo-parietal, (pre-)motor, and superior temporal regions, with different patterns according to the task. Notably, increased connectivity from the right superior temporal cortex to the left sensorimotor cortex correlated with faster naming times in the listening-for-speaking task. These findings suggest that atypical speech-motor functioning in stuttering impact also speech perception, especially in situations requiring articulatory alertness, and highlight the involvement of frontal and (pre-) motor regions in normal conditions in CTS. **Key Objectives and Methods** * The study aimed to analyze CTS in individuals with developmental stuttering (DS) compared to typically fluent adults (TFA). * Participants engaged in two listening tasks: listening-only and listening-for-speaking, the latter involving an upcoming speech production. * Researchers measured coherence in the theta range (3-5 Hz) and cortical connectivity. # Main Findings **Theta Coherence**: * AWS showed reduced theta coherence in the listening-for-speaking task, particularly in the left inferior parietal/temporo-parietal cortex and right premotor/supplementary motor regions. * Theta rhythm is crucial for syllabic grouping and speech perception. **Cortical Connectivity**: * AWS exhibited weaker inter-hemispheric connectivity between auditory, frontal, and sensorimotor regions in both tasks. * Notably, reduced connectivity was associated with slower speech response times (RTs) in AWS during the listening-for-speaking task. **Behavioral Correlations**: * A significant negative correlation was found between connectivity strength (right STG → left SM cortex) and RTs, suggesting that better connectivity facilitates faster speech responses. #### Theoretical Implications * **Speech-Brain Synchronization**: * CTS involves aligning brain frequencies with speech signals, supported by speech-motor regions. * In AWS, impaired CTS during speech listening and production transitions may stem from unstable speech-motor planning. * **Region-Specific Impact**: * The left inferior parietal cortex and right premotor areas are vital for speech-motor coordination. * These regions are often dysfunctional in DS, affecting the neural circuits necessary for efficient speech processing. * **Theta Rhythm and Syllable Processing**: * Theta rhythms correspond to the syllabic rate and are essential for speech perception and motor programming. * In DS, the inability to efficiently track these rhythms reflects broader speech-motor system instability. #### Connectivity Patterns * **Listening-Only Task**: * Weaker connectivity from the left STG to the right IFG and right IFG to left IPL/TPJ was observed. * **Listening-for-Speaking Task**: * Weaker connectivity from the right STG to left SM regions and left IPL/TPJ was found. * Efficient connectivity between these regions is crucial for smooth transitioning between listening and speaking. #### Behavioral and Clinical Significance * **Speech Response Times**: * Slower RTs in AWS highlight the impact of connectivity deficits on speech processing efficiency. * Correlations between connectivity strength and RTs suggest that stronger neural connections support better speech production. * **Impact on Everyday Life**: * Suboptimal CTS in DS may affect not just speech production but also speech perception and intelligibility. * This could lead to more effortful language comprehension, especially in conversational contexts. ## Conclusions The study provides evidence that developmental stuttering involves significant impairments in CTS, particularly during tasks requiring speech production. These impairments are linked to weaker inter-hemispheric connectivity and slower speech response times, suggesting that enhancing neural connectivity could be a potential therapeutic target for improving speech fluency in DS. Further research is needed to explore the broader implications of these findings on speech comprehension and everyday communication in individuals with DS. The present work suggests that CTS recruits (pre-)motor regions and regions responsible for sensorimotor integration, as well as auditory regions, supporting the view proposing an interaction between these networks also in speech/language perception ([**Pickering & Garrod, 2013**](https://www.biorxiv.org/content/10.1101/2024.02.23.581767v1#ref-86); [**Skipper et al., 2017**](https://www.biorxiv.org/content/10.1101/2024.02.23.581767v1#ref-97)), in addition to be instrumental in orchestrating successful speech production ([**Guenther, 2016**](https://www.biorxiv.org/content/10.1101/2024.02.23.581767v1#ref-45); [**Hickok et al., 2011**](https://www.biorxiv.org/content/10.1101/2024.02.23.581767v1#ref-48)). This process seems to work less efficiently in DS, especially when additional neural resources are needed for managing listening-for-speaking conditions, as usually happens in more ecological communicative situations ([**Neef & Chang, 2024**](https://www.biorxiv.org/content/10.1101/2024.02.23.581767v1#ref-80)). A better understanding of CTS processes in DS under various circumstances may be informative for improving rehabilitation solutions for stuttering.