The right inferior frontal gyrus and poststroke aphasia: a follow-up investigation.

BACKGROUND AND PURPOSE: Recently, a combined repetitive transcranial magnetic stimulation (rTMS) and activation positron emission tomography (PET) study showed essential language function of the right inferior frontal gyrus (IFG) in some right-handed acute poststroke aphasics. We reexamined these patients in the chronic phase to test whether the right IFG remained essential for language performance. METHODS: We reexamined 9 male right-handed patients, age 41 to 75 years, with aphasia 8 weeks after left hemispheric stroke. rTMS was performed over the maximum activation within the left and right IFG as defined by (15)[O]water PET to interfere with language function. A positive rTMS effect was defined as increased reaction time latency or error rate in the semantic task relative to no stimulation. RESULTS: PET activations of the IFG were observed on the left (2 patients) and bilaterally (7). During rTMS interference over the left IFG, all patients had positive TMS effects, indicating that the left IFG remained essential. Stimulation over the right IFG yielded positive rTMS effects in 2 patients with persisting right IFG activation. Two patients with positive rTMS effects over the right side in the initial study did not show these effects at follow-up. Language performance improved in all patients. CONCLUSIONS: Successful regeneration from poststroke aphasia seems to depend more on the integration of available language-related brain regions than on recruiting new brain regions during the rehabilitation process. Restoration of the left hemisphere network seems to be more effective, although in some cases, right hemisphere areas are integrated successfully.

Early electroencephalography in acute ischemic stroke: prediction of a malignant course?

OBJECTIVE: In patients with large middle cerebral artery (MCA) infarction space occupying brain edema may lead to a malignant course with up to 80% mortality under conservative treatment. As interventional treatment strategies must be started before the deterioration occurs predictors of a malignant course are necessary. PATIENTS AND METHODS: This study reports on the results of early electroencephalography (EEG) within 24h after onset of stroke in 25 patients suffering a large MCA infarct (12 patients with a malignant and 13 with a non-malignant course). EEG analysis was performed according well-established indicators for focal as well as global changes. RESULTS: Our findings indicate that the absence of delta activity and the presence of theta and fast beta frequencies within the focus predict a benign course (p < 0.05), whereas diffuse generalized slowing and slow delta activity in the ischemic hemisphere may point to a malignant course. CONCLUSION: This study shows that in patients suffering from large MCA infarction early EEG delivers useful information to select those patients who develop malignant edema.

From the left to the right: How the brain compensates progressive loss of language function.

In normal right-handed subjects language production usually is a function oft the left brain hemisphere. Patients with aphasia following brain damage to the left hemisphere have a considerable potential to compensate for the loss of this function. Sometimes, but not always, areas of the right hemisphere which are homologous to language areas of the left hemisphere in normal subjects are successfully employed for compensation but this integration process may need time to develop. We investigated right-handed patients with left hemisphere brain tumors as a model of continuously progressive brain damage to left hemisphere language areas using functional neuroimaging and transcranial magnetic stimulation (TMS) to identify factors which determine successful compensation of lost language function. Only patients with slowly progressing brain lesions recovered right-sided language function as detected by TMS. In patients with rapidly progressive lesions no right-sided language function was found and language performance was linearly correlated with the lateralization of language related brain activation to the left hemisphere. It can thus be concluded that time is the factor which determines successful integration of the right hemisphere into the language network for compensation of lost left hemisphere language function.

Role of the contralateral inferior frontal gyrus in recovery of language function in poststroke aphasia: a combined repetitive transcranial magnetic stimulation and positron emission tomography study.

BACKGROUND AND PURPOSE: Functional neuroimaging studies have demonstrated right inferior frontal gyrus (IFG) activation in poststroke aphasia. It remains unclear whether this activation is essential for language performance. We tested this hypothesis in a positron emission tomography (PET) activation study during a semantic task with repetitive transcranial magnetic stimulation (rTMS) on right-handed patients experiencing poststroke aphasia and examined whether rTMS stimulation over the right and left IFG would interfere with language performance. METHODS: Eleven patients with left-sided middle cerebral arterial infarction, 50 to 75 years of age, were tested with the Aachen Aphasia Test Battery and underwent (15)O-H2O PET activation during a semantic task within 2 weeks after stroke. PET activation images were coregistered to T1-weighted MRIs. Stimulation sites were determined on renderings of head and brain over the maximum activation within left and right IFG. rTMS was performed with 20% maximum output (2.1 T), 10-s train duration, at 4 Hz frequency. A positive rTMS effect was defined as an increased reaction time latency or error rate in the semantic task. RESULTS: PET activations of the IFG were observed on the left (3 patients) and bilaterally (8 patients). Right IFG stimulation was positive in 5 patients with right IFG activation, indicating essential language function. In a verbal fluency task, these patients had a lower performance than patients without right-sided TMS effect. CONCLUSIONS: In some poststroke aphasics, right IFG activation is essential for residual language function. However, its compensatory potential seems to be less effective than in patients who recover left IFG function. These results suggest a hierarchy in recovery from poststroke aphasia and a (limited) compensatory potential of the nondominant hemisphere.

Neuroimaging-guided rTMS of the left inferior frontal gyrus interferes with repetition priming.

Neuroimaging studies of right-handed normal volunteers under semantic word generation tasks have consistently reported left lateralized activation of the anterior inferior frontal gyrus (ifg) which decreased during task repetition. This repetition-related activation decrease has been interpreted as the neurophysiological correlate of repetition priming, a mechanism of implicit memory for initial semantic processing. We interfered with left lateralized ifg activation, as identified by O-15-water PET activation, using repetitive transcranial magnetic stimulation (rTMS) in five right-handed male normal subjects, once using new (unprimed) nouns and once using known (primed) nouns for the procedure. All five subjects exhibited clear left lateralized activations of the triangular part of the left ifg in the PET studies. In all subjects, reaction time latencies were significantly longer during rTMS over the activation sites in the left ifg as compared to latencies off stimulation. Latencies were not affected during stimulation of the right ifg or over the vertex. These effects were observed within the group and in each individual, only if lists of primed nouns were used in the verb-generation task. In conclusion, these results demonstrate that the anterior part of the left ifg is not only involved in semantic processing, but is also essential for repetition priming on semantic tasks since successful interference with rTMS was only observed if lists of primed words were used for the generation task.

Essential language function of the right hemisphere in brain tumor patients.

Neuroimaging studies of language networks in patients with brain lesions of the left language-dominant hemisphere have shown activation in the right inferior frontal gyrus (IFG). We tested the functional relevance of right IFG activation using neuroimaging-guided repetitive transcranial magnetic stimulation (rTMS) to disturb language function over bilateral IFG in right-handed patients with brain tumors and controls. All subjects were susceptible to TMS over the left IFG. In patients, this susceptibility correlated with left-sided the degree of language lateralization to the left. Those patients with lowest dominance were also susceptible to right-sided TMS proving relevant language function of the right IFG.

Functional imaging in the assessment of capability for recovery after stroke.

After an ischaemic lesion preserved components of a functional network are utilized for recovery from neurological defects. The hierarchy of the individual parts within the damaged network, however, determines the quality of the outcome. This could be clearly demonstrated for the complex network of language ability, for which the left temporal region plays an integrative role: only if the left temporal regions are morphologically preserved and can be reactivated in imaging studies of speech performance was the outcome of poststroke aphasia satisfying. In brain functions with a less pronounced hemispheric specialization the effect of disturbed centres in the dominant hemisphere might be less accentuated. Functional neuroimaging studies might help to estimate prognoses of functional outcome in early states after stroke and to evaluate the efficiency of strategies of rehabilitative measures and of adjuvant drug therapy.