The precentral insular cortical network for speech articulation

Apraxia of speech is a motor disorder characterized by the impaired ability to coordinate the sequential articulatory movements necessary to produce speech. The critical cortical area(s) involved in speech apraxia remain controversial because many of the previously reported cases had additional aphasic impairments, preventing localization of the specific cortical circuit necessary for the somatomotor execution of speech. Four patients with "pure speech apraxia" (i.e., who had no aphasic and orofacial motor impairments) are reported here. The critical lesion in all four patients involved, in the left hemisphere, the precentral gyrus of the insula (gyrus brevis III) and, to a lesser extent, the nearby areas with which it is strongly connected: the adjacent subcentral opercular cortex (part of secondary somatosensory cortex) and the most inferior part of the central sulcus where the orofacial musculature is represented. There was no damage to rostrally adjacent Broca's area in the inferior frontal gyrus. The present study demonstrates the critical circuit for the coordination of complex articulatory movements prior to and during the execution of the motor speech plans. Importantly, this specific cortical circuit is different from those that relate to the cognitive aspects of language production (e.g., Broca's area on the inferior frontal gyrus).