Motor somatotopy of supralaryngeal and laryngeal articulators
Résumé
Introduction: Since electrocortical stimulation studies by Penfield and colleagues, there is evidence that the human body is functionally represented in a somatotopic manner on several cerebral structures. Although a number of functional magnetic resonance imaging (fMRI) studies dealt with body part locations of hand, foot and face/mouth on the motor cortex, few tried a fine discrimination of orofacial articulator movements. The present fMRI study is the first to compare, within the same participants, neural networks related to lip, jaw, larynx and tongue movements and to try discriminating underlying representations within the motor cortex. Methods: 13 native french right-handed participants lied supine in a Bruker 3T imager. The tasks consisted in protruding lips, in lowering jaws, doing a retroflex tongue movement and vocalizing the vowel /i/. For lips, jaw and tongue movements, visual instructions were presented in a single run of 13 minutes and in a pseudorandom order. The /i/ vowel was produced by the same participants but in a second functional run. In order to minimize movement-related imaging artifacts, a sparse sampling acquisition paradigm was used where image acquisition occured with a 4-6s delay regarding task performance. In total, 72 functional images were acquired (18 per task plus 18 resting trials). Data analyses: Using the SPM5 sofware, a random-effect group analysis was performed using repeated-measures ANOVA (p < .05, FWE corrected, clusters superior to 100 voxels). At the individual level, region of interest analyses were conducted within the primary motor cortex for lip, jaw, larynx and vowel movements. Coordinates of both centers of gravity and maximum activity peaks of activated motor clusters were estimated for each participant and entered into repeated-measure ANOVAs with the articulators and the hemisphere as within-subject factors. Results: Group analysis revealed strong bilateral activations within the sensorimotor cortex as well as within the supplementary motor area and the anterior cingulate gyrus, the inferior frontal gyrus, the insula, the inferior parietal lobule, the putamen, the thalamus and the cerebellum. At the individual level, a dorso-ventral bilateral somatotopic organization of lip, jaw, larynx and tongue articulators, respectively, was detectable in the sensorimotor cortex and could be spatially discriminated for the great majority of the participants. Discussion: Our results show largely overlapping but distinct networks for lips, jaw and the tongue movements classically involved in motor control. Consistent with previous studies, a sequential dorso-ventral somatotopy somatotopy of speech articulators was observed within the motor cortex. Furthermore, these results demonstrate the possibility to finely differentiate representations of supralaryngeal and laryngeal articulator's movements even at the individual level and are of particular interest for studies investigating speech motor control and disorders as well as cortical plasticity after neurosurgery.