Neuronal bases of "apraxia of speech"

Apraxia of speech (AOS) has been reported to be closely related to frontal lesions in the language dominant hemisphere. However, the exact location of the lesion that causes AOS remains to be determined. We reviewed reported cases of pure AOS in which lesions were clearly demonstrated by MRI, CT or autopsy studies. Twenty-one cases had unilateral lesions in the left cerebral hemisphere, and all except one demonstrated lesions in the inferior part of the left precentral gyrus. In contrast, another study examining aphasic patients with AOS as one element of aphasic symptoms had a left insular lesion in common. We examined patients with left frontal and insular lesions to clarify the region crucial to causing AOS. All patients with AOS had lesions in the left precentral gyrus, while patients without AOS demonstrated lesions including the insular cortex but not the precentral gyrus. Language mapping using electric cortical stimulation during operation revealed that stimulation to the inferior part of the left precentral gyrus caused speech arrest or delayed responses. These impairments were not language specific but were related to the tongue motor area or negative motor areas. These findings suggest that the left precentral gyrus may strategically act as a node to control complex orolingual movements including speech.