Brain regions involved in motor control of speech

1. Introduction Human speech production requires a complex coordination of motor control for respiration, vocalization, and articulation. The neural mechanisms for speaking have been studied since the 17th century, yet many questions remain in determining the brain regions involved in speech production. In evidence of this, recent functional brain imaging studies examining normal subjects' articulation have resulted in inconsistent activation patterns. The regions that are engaged in speech articulation spread over the cortical and subcortical areas. Within the cerebral cortex, the sensorimotor area is commonly observed to be active, while other areas often show disagreements. Broca's area has long been believed to be involved in speech articulation. Localized infarction within Broca's area, however , does not always cause classical Broca's aphasia [1], and some doubt has been posed with respect to the role of Broca's area in executing articulatory tasks without language processing. In fact, no activation was detected in this area in an experiment using positron emission tomography (PET) during articulation of a simple phrase [2] or during repetitions of heard words [3]. A lesion-based analysis by Dronkers [4] has shown that the left anterior insula is associated with the motor coordination of articulation. Wise et al. [3] measured brain activity while subjects repeated heard nouns, and showed activity of the left anterior insula. No activation of the insula during articulation was detected, however, during articulation in a PET study [2] or a functional magnetic resonance imaginig (fMRI) study [5]. Other areas besides the cortex are also known to be involved in speech articulation. Clinical data of speech disorders in patients with cerebellar lesions or Parkinson's disease suggest that the cerebellum and the brain stem are engaged in the motor control of speech [6–8]. Ackermann et al. [9] showed activation of the right superior cerebellum during recitations of the names of the months by normal subjects. Wise et al. [3] also showed bilateral activation of the rostral paravermal cerebellum in repetitions of heard nouns. In producing syllables at a self-paced speaking rate, however, few significant responses of the cerebellum were detected [5]. There are not many studies regarding the activities of the brainstem and midbrain during speech. Wise et al. [3] reported activation of the dorsal brainstem during articulation. As far as we know, there have been no reports of the detection