Functional and Quantitative MRI Mapping of Somatomotor Representations of Human Supralaryngeal Vocal Tract.

Daniel Carey,Frederic Dick,Martina F Callaghan,Saloni Krishnan,Martin I Sereno

doi:10.1093/cercor/bhw393

Abstract

Speech articulation requires precise control of and coordination between the effectors of the vocal tract (e.g., lips, tongue, soft palate, and larynx). However, it is unclear how the cortex represents movements of and contact between these effectors during speech, or how these cortical responses relate to inter-regional anatomical borders. Here, we used phase-encoded fMRI to map somatomotor representations of speech articulations. Phonetically trained participants produced speech phones, progressing from front (bilabial) to back (glottal) place of articulation. Maps of cortical myelin proxies (R1 = 1/T1) further allowed us to situate functional maps with respect to anatomical borders of motor and somatosensory regions. Across participants, we found a consistent topological map of place of articulation, spanning the central sulcus and primary motor and somatosensory areas, that moved from lateral to inferior as place of articulation progressed from front to back. Phones produced at velar and glottal places of articulation activated the inferior aspect of the central sulcus, but with considerable across-subject variability. R1 maps for a subset of participants revealed that articulator maps extended posteriorly into secondary somatosensory regions. These results show consistent topological organization of cortical representations of the vocal apparatus in the context of speech behavior.

Highlights

The supralaryngeal vocal tract (SVT) comprises a complex set of sensory surfaces and motor effectors that, in primates, are represented within and across multiple cortical areas
We found a consistent topological map of place of articulation, spanning the central sulcus and primary motor and somatosensory areas, that moved from lateral to inferior as place of articulation progressed from front to back
We used a phase-encoded fMRI paradigm to noninvasively map the vocal tract articulators, as subjects produced a range of speech phones that varied systematically in their place of articulation

Summary

Introduction

The supralaryngeal vocal tract (SVT) comprises a complex set of sensory surfaces and motor effectors that, in primates, are represented within and across multiple cortical areas. Evidence from nonhuman primates has shown that complex, multi-effector forelimb postures (Graziano et al 2002a, 2002b; Aflalo and Graziano 2006b; Overduin et al 2012) and oral effector movements (Graziano et al 2002a) occur following electrical stimulation of M-I neurons Such movements typically occur toward specific locations in space (Graziano et al 2005; Overduin et al 2012), are evoked at timescales relevant to performing complex actions (Graziano et al 2002a), and largely fall within the set of ethological activities relevant to the animal (e.g., feeding and self-defense; Graziano and Cooke 2005). Speech necessitates the contact between the SVT articulators, and complex synergies of effector movements, careful planning before and following each articulation, and control of airflow mechanisms

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