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Electrophysiological correlates of speech modulation and syllable complexity in neurotypical and brain damaged speakers to disentangle motor speech encoding mechanisms
Poster Session E, Friday, October 2, 11:00 am - 1:00 pm, Wangari Maathai
Bryan Sanders1, Monica Lancheros1, Marina Laganaro1; 1University of Geneva
Motor speech encoding (MSE) — i.e., the cognitivo-motor transformation of language into speech production — is an emerging field of investigation trying to account for the huge panel of speech phenomena observed in verbal exchanges. While the literature globally agrees that MSE can be subdivided into two subprocesses (Fougeron et al., 2025), the actual mechanistical interplay between these processes remains an open debate. Consequently, core MSE questions such as the size of the gestural scores (Laganaro, 2019), the way a speaker increases vocal loudness (Tourville et al., 2011) and comorbidity in motor speech disorders (Van der Merwe, 2021) cannot yet be clearly characterized relying on current theoretical frameworks. The present study aims to clarify how speakers transform an abstract phonological input into articulatory trajectories by contrasting the electrophysiological (EEG) correlates of neurotypical and brain damaged speakers in two distinct conditions. Participants produced disyllabic pseudowords varying in syllable complexity — CV versus CCV structures of the first syllable — while producing speech in two different modes of production — standard speech versus loud speech — during a delayed production task. Behavioral analyses were coupled with waveform and microstates analyses carried out on event related potential (ERP) signals on a group of 22 neurotypical speakers and 11 speakers suffering from either apraxia of speech (n=7) or hypokinetic dysarthria (n=4) using a single case approach. Behavioral results on group level analyses demonstrated increased latencies for the loud speech modality and reduced latency for producing CCV structures. Waveform analyses on neurotypical speakers showed that differences in amplitudes were more extended for the speech modulation contrast in comparison to the syllable complexity contrast. Topographical analyses showed that the encoding of varying syllable complexity and loud speech properties were bound to specific time windows respectively close and far from the vocal onset. Single case comparison of each patient with the ERP signals of the neurotypical group is currently ongoing but preliminary results follow the same pattern observed in neurotypical speakers; a long time period of electrophysiological differences with an early onset associated to speech modulation and a short time period with a late onset close to the beginning of the acoustic signal in the syllable complexity condition. Although preliminary, the present results directly challenge several assumptions from the literature. First, a behavioral encoding cost was expected for demanding modalities such as CCV structure and speaking louder than usual. While loud utterances entailed the expected increase in latencies, participants were actually faster to produce pseudowords with CCV structure compared to CV structure. This unexpected result was previously noticed (Jouen et al., 2023) and would thus need further investigation to provide an informed mechanistical account. Furthermore, it was expected that the EEG signature for syllable complexity would be observable in an earlier time period compared to speech modulation. Nonetheless, this hypothesis was rejected from both group level and single case analyses independently of the type of neuropathology. Further analyses on these findings may provide meaningful insights regarding the interplay of the underlying subprocesses of motor speech encoding.
Topic Areas: Speech Motor Control,