Understanding how the motor cortex participates in active inference of speech perception and choice is critical for understanding human language intelligence, which may inspire the next generation of AI.
Motor cortex helps restoring auditory speech representation in noise. Inspired by archaeology, the “relics” of syllables (i.e., auditory representations) are placed on the “land” of the auditory cortex. The “dust” (i.e., noise interfering with the input sound signals) blurs the auditory representations of syllables; and the way the cerebral speech motor system (the human head) is involved in auditory processing is similar to that of an archaeologist: matching its stored “motor templates” with the buried syllable representations, top-down assisting the auditory cortex to brush off the dust, and restoring the original content produced by the speaker. Image Credit: WANG Shiyu
Three major concerns about the involvement of the bilateral motor cortices in speech perception, however, remain unsolved. Is the laryngeal motor cortex (LMC) active in both speech perception and production? How do bilateral motor cortices collaborate when the task is difficult? What steps of the perceptual decision-making process are the bilateral motor cortices modulating?
A research team led by Dr. Yi Du at the Institute of Psychology of the Chinese Academy of Sciences discovered in a study published in Nature Communications that the bilateral LMC in the human brain is causally involved in various phases of speech perception decision-making, and that the LMC engagement is left-dominant and especially helps auditory processing in situations with perceptual challenges.
Two experiments were carried out by the researchers. In Experiment 1, they delivered repetitive transcranial magnetic stimulation (rTMS) and theta-burst stimulation (TBS) to the left/right motor cortex of healthy Mandarin adult speakers to see if the categorical perceptual decision of lexical tone and plosive consonant with/without noisy background was modulated.
Participants conducted a functional magnetic resonance imaging (fMRI) pretest in which they performed phonation and tongue movement activities to locate the production-related dorsal LMC (dLMC) and TMC.
The researchers next used two distinct data-analysis pipelines to evaluate the impact of TBS modulation on dLMC on behavioral responses in Experiment 2. They investigated modulations on the slope of psychometric curves to discover variations in perceptual sensitivity.
They used the hierarchical Bayesian estimate of the drift-diffusion model (HDDM) to determine which latent dynamic decision processes would be adjusted to evaluate changes at certain phases of perceptual judgment.
cTBS upon the left dLMC inhibited tone perception in noise and inhibited consonant perception in both quiet and noise, while cTBS upon the right dLMC inhibited consonant perception in noise but not in quiet and had no effect on tone perception, according to psychometric curve slope analyses.
HDDM analyses revealed that cTBS significantly widened the decision-making range for all conditions, with the exception of tone perception in quiet by left dLMC stimulation; cTBS upon left dLMC affected evidence accumulation rates, but right dLMC stimulation was unable to have a similar impact; and cTBS upon both left and right dLMC affected response biases for consonant perception in noise.
The findings suggested that the human LMC is causally involved in speech perception as well as speech production since they showed effector-specific involvement of the bilateral dLMC in the perceptual perception of both lexical tone and voicing of plosive consonants.
As the left dLMC predominates and the right counterpart is only important in difficult tasks, they also offered evidence for the redundancy and functional reorganization of neural networks. Furthermore, the hemisphere and level of task difficulty determine whether particular perceptual steps are controlled by the dLMC.
In speech perception, our articulatory motor cortex acts like a denoiser that predicts the upcoming words by simulating the embedded motor gestures. Our study provides important empirical evidence to support that bilateral laryngeal motor cortices, the motor subregions essential for voicing and pitch control, are also parts of such denoiser systems.”
Dr. Yi Du, Study Corresponding Author and Professor, Institute of Psychology, Chinese Academy of Sciences
The fundamental processes and temporal dynamics of bilateral motor activation in speech-perceptual decision-making were better understood in this study. It has implications for the creation of more resilient AI systems with dynamical adaptivity as well as clinical translational research for the rehabilitation of speech disorders.
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Journal reference:
Liang, B., et al. (2023). Bilateral human laryngeal motor cortex in perceptual decision of lexical tone and voicing of consonant. Nature Communications. doi.org/10.1038/s41467-023-40445-0
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