Apraxia of speech (AOS)/anarthrie is a motor speech disorder characterized by core symptoms of distorted articulation and segmentation of syllables. However, little is understood with regard to the nature of AOS. We explained the neuropsychological and neuroanatomical features of patients with stroke-induced pure AOS (sAOS), patients with primary progressive apraxia of speech (PPAOS), and normal control adults. Based on the previous reports, all patients with sAOS could be classified into four subtypes: type I with prominent distorted articulation; type II with prominent segmentation of syllables; type III with similarly prominent distorted articulation and segmentation of syllables; or type IV with prominent distorted articulation and no signs of segmentation of syllables. In the investigation of PPAOS using the same methods as the above report, PPAOS with prominent distorted articulation. However, it was only PPAOS that the reduction of speech rate was prominent even though segmentation of syllables was not remarkable. The lesions of a patient with type I was located in the left precentral gyrus, type II in both the left precentral gyrus and left premotor cortex, type III in the white matter of the left periventricular region, type IV in the white matter of the left periventricular region and left striatum. Patients with PPAOS demonstrated significant reduction of regional cerebral blood flow in the left superior premotor cortex and bilateral supplementary motor area. We speculate that a focus on the contrast between distorted articulation, segmentation of syllables, and speech rate could be useful for sub-classification of AOS.
Supplementary motor area (SMA) is regarded as including two separate areas; preSMA and SMAproper. Both preSMA and SMAproper have the relationship with language function, in particular preSMA has specific fiber tracts between prefrontal area and seems to have strong relation with language which has been evidenced by fMRI studies and clinical reports. Left medial frontal lesions cause supplementary motor area aphasia with characteristic symptoms: reduced spontaneous speech with prominent contrast of preserved ability of confrontation naming and severe impairment of word fluency. Right medial frontal lesions might cause affective prosody impairment. Regarding left medial parietal lesions, some clinical reports indicate a possibility of the relation between medial parietal region and language comprehension ability. Orbitofrontal lesion has been reported to show narrative impairments.
We need to understand aphasia in terms of two points of view. One is the viewpoint of system of language, and the other is the viewpoint of brain function. Language systems are hierarchically constructed, for instance, acoustic system (hearing and speech), phonetic system (discrimination of language sounds and articulation control), phonemic system and lexical/semantic system. Language impairments can be developed of each system as language symptoms. And there are corresponding lesion sites in the brain for each elementary language symptoms such as anarthrie/apraxia of speech, impairment of discrimination of language sound (word deafness), phonemic paraphasia, word comprehension impairment, word retrieval impairment and so on. Classical aphasia classification are vascular syndromes and can be explained as the syndromes made of the elementary symptoms.
The author investigated the implication of the frontal lobe in language comprehension using so-called pointing task (referring the task to choose and point to the proper object according to the presented word) and the sentence order task (referring the task to react according to the presented order by a sentence) , and the test of syntactic processing in aphasia. The results show as followings: 1. The deficit in the pointing task has relation to the lesion in the left middle frontal gyrus. The deficit in the pointing task due to frontal lobe damage might not be stemmed from the impairment of semantic process, but from the process in the way of choosing the objects according the target word. 2. The deficit in the sentence order task is closely related to the lesion in Brocaʼs area. The patients with frontal lobe lesion has impairment in this kind of task due to deficit of switching the view point when they face an unexpected sentence patterns.