We assessed brain activation of nine normal right-handed volunteers in a positron emission tomography study designed to differentiate the functional anatomy of the two major components of auditory comprehension of language, namely phonological versus lexico-semantic processing. The activation paradigm included three tasks. In the reference task, subjects were asked to detect rising pitch within a series of pure tones. In the phonological task, they had to monitor the sequential phonemic organization of non-words. In the lexico-semantic task, they monitored concrete nouns according to semantic criteria. We found highly significant and different patterns of activation. Phonological processing was associated with activation in the left superior temporal gyrus (mainly Wernicke's area) and, to a lesser extent, in Broca's area and in the right superior temporal regions. Lexico-semantic processing was associated with activity in the left middle and inferior temporal gyri, the left inferior parietal region and the left superior prefrontal region, in addition to the superior temporal regions. A comparison of the pattern of activation obtained with the lexico-semantic task to that obtained with the phonological task was made in order to account for the contribution of lower stage components to semantic processing. No difference in activation was found in Broca's area and superior temporal areas which suggests that these areas are activated by the phonological component of both tasks, but activation was noted in the temporal, parietal and frontal multi-modal association areas. These constitute parts of a large network that represent the specific anatomic substrate of the lexico-semantic processing of language.
We have previously shown bilateral activation of motor pathways and the recruitment of additional motor areas in studies of groups of patients with recovery from motor stroke. We have now developed a new positron emission tomographic technique to measure the changes in regional cerebral blood flow elicited during a motor task in individual patients, relative to the cerebral activation found in normal subjects. The patterns of cerebral activation in each of 8 individual patients with capsular lesions of the pyramidal tract and complete recovery from hemiplegia are described by comparison with the pattern found in a representative sample of 10 normal subjects. We found a large ventral extension of the hand field of the contralateral (sensori)motor cortex in all patients with lesions of the posterior limb of the internal capsule. Greater activation than in normal subjects was found in variable combinations of the supplementary motor areas, the insula, the frontal operculum, and the parietal cortex. Structures belonging to motor pathways ipsilateral to the recovered limb were also more activated in the patients than in normal subjects. However, additional activation of the ipsilateral (sensori)motor cortex was only found in the 4 patients who exhibited associated movements of the unaffected hand when the recovered hand performed the motor task. We conclude that recovery from motor stroke due to striatocapsular damage is associated with individually different patterns of functional reorganization of the brain. These patterns are dependent on the site of the subcortical lesion and the somatotopic organization of the pyramidal tract, both of which may determine the precise potential for recovery of limb function following this type of brain injury.
PET activation studies identify significant local changes in regional cerebral blood flow (rCBF) in contrasts of behavioural tasks with control states, and these local changes identify net changes in local synaptic activity. A number of studies on word retrieval have all demonstrated left frontal (dorsolateral and medial) involvement in the task. However, there have been differences in the responses observed in the left temporal lobe, with variously a deactivation (significant decrease in rCBF), no response and an activation (significant increase in rCBF). In the four studies described here, we have examined word (verbs and nouns) retrieval contrasted with a number of different control states. The studies confirmed extensive activation of the left dorsolateral prefrontal cortex and, medially, the anterior cingulate cortex and the supplementary motor area (SMA). Activations of the left posterior temporal lobe and the inferior parietal lobe were consistently demonstrated when word retrieval was contrasted with a rest state. Contrasts with other single word tasks controlled out the activation in the perisylvian part of the left posterior temporal lobe, suggesting a role for this region in lexical processing. The left inferolateral temporal cortex and the posterior part of the inferior parietal lobe were only activated by word retrieval, particularly verbs. It is proposed that these activated regions reflect access to semantic fields.
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