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This Article Full Text Full Text (PDF) All Versions of this Article: 102/1/159    most recent 90894.2008v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Sowman, P. F. Articles by Nordstrom, M. A. PubMed PubMed Citation Articles by Sowman, P. F. Articles by Nordstrom, M. A.

Asymmetric Activation of Motor Cortex Controlling Human Anterior Digastric Muscles During Speech and Target-Directed Jaw Movements

¹Discipline of Physiology, School of Molecular and Biomedical Sciences and ²Research Centre for Human Movement Control, The University of Adelaide, Adelaide, South Australia, Australia; and ³Department of Fixed Prosthodontics, School of Dentistry, Aichi-Gakuin University, Nagoya, Japan

Submitted 8 August 2008; accepted in final form 3 May 2009

Like most of the cranial muscles involved in speech, the trigeminally innervated anterior digastric muscles are controlled by descending corticobulbar projections from the primary motor cortex (M1) of each hemisphere. We hypothesized that changes in corticobulbar M1 excitability during speech production would show a hemispheric asymmetry favoring the left side, which is the dominant hemisphere for language processing in most strongly right handed subjects. Fifteen volunteers aged 24.5 ± 5.3 (SD) yr participated. All subjects were strongly right handed as reported by questionnaire. A surface electromyograph (EMG) was recorded bilaterally from digastrics and jaw movement detected by an accelerometer attached to a lower incisor. Focal transcranial magnetic stimulation (TMS) was used to assess corticomotor excitability of the digastric representation in M1 of both hemispheres during four tasks: 1) static isometric contraction of digastrics; 2) speaking a single word; 3) visually guided, nonspeech jaw movement that matched the jaw kinematics recorded during task 2; and 4) reciting a sentence. Background EMG was well matched in all tasks and jaw kinematics were similar around the time of the TMS pulse for tasks 2–4. TMS resting thresholds and digastric muscle-evoked potential (MEP) size during isometric contraction did not differ for TMS over left versus right M1. MEPs elicited by TMS over left, but not right M1 increased in size during speech and nonspeech jaw movement compared with isometric contraction. We conclude that left corticobulbar M1 is preferentially engaged for descending control of digastric muscles during speech and the performance of a rapid jaw movement to match a target kinematic profile.