Recent functional imaging studies demonstrated that both the left and right supramarginal gyri (SMG) are activated when healthy right-handed subjects make phonological word decisions. However, lesion studies typically report difficulties with phonological processing after left rather than right hemisphere damage. Here, we used a unique dual-site transcranial magnetic stimulation (TMS) approach to test whether the SMG in the right hemisphere contributes to modality-independent (i.e., auditory and visual) phonological decisions. To test task-specificity, we compared the effect of real or sham TMS during phonological, semantic, and perceptual decisions. To test laterality and anatomical specificity, we compared the effect of TMS over the left, right, or bilateral SMG and angular gyri. The accuracy and reaction times of phonological decisions were selectively disrupted relative to semantic and perceptual decisions when real TMS was applied over the left, right, or bilateral SMG. These effects were not observed for TMS over the angular gyri. A follow-up experiment indicated that the threshold-intensity for inducing a disruptive effect on phonological decisions was identical for unilateral TMS over the right or left SMG. Taken together, these findings provide converging evidence that the right SMG contributes to accurate and efficient phonological decisions in the healthy brain, with no evidence that the left and right SMG can compensate for one another during TMS. Our findings motivate detailed studies of phonological processing in patients with acute or long-term damage of the right SMG.M any previous functional imaging studies have shown that the left and right supramarginal gyri (SMG) are activated when right-handed participants make decisions about the sounds of words (i.e., their phonology) compared with decisions about their meanings (i.e., their semantics) (1-4). However, the functional significance of right SMG activation is unclear because lesion studies have reported phonological difficulties following left rather than right temporo-parietal lesions (5-8). Consequently, anatomical models of phonological processing have included left but not right parietal cortex (9, 10). The present study was designed to address the discrepancy between functional imaging and lesion studies. More specifically, we examined how "online" transcranial magnetic stimulation (i.e., TMS during a task) over the left and right SMG influences phonological word processing in healthy subjects (Fig. 1). We used the neurodisruptive effect of TMS to distinguish between three alternative hypotheses to explain right SMG activation with phonological processing.Hypothesis 1 is that right SMG only contributes to the speed but not the accuracy of phonological decisions. Consequently, right SMG lesions have a subtle effect on phonological processing that might be missed unless reaction times were measured. In this case, we expect a selective effect of right SMG TMS on reaction times in the healthy brain without affecting error rates.Hypothesis 2 i...

Kinesthetic motor imagery and actual execution of movements share a common neural circuitry. Functional magnetic resonance imaging was used in 12 right-handed volunteers to study brain activity during motor imagery and execution of simple and complex unimanual finger movements of the dominant and the nondominant hand. In the simple task, a flexible object was rhythmically compressed between thumb, index and middle finger. The complex task was a sequential finger-to-thumb opposition movement. Premotor, posterior parietal and cerebellar regions were significantly more active during motor imagery of complex movements than during mental rehearsal of the simple task. In 10 of the subjects, we also used transcranial magnetic brain stimulation to examine corticospinal excitability during the same motor imagery tasks. Motor-evoked potentials increased significantly over values obtained in a reference condition (visual imagery) during imagery of the complex, but not of the simple movement. Imagery of finger movements of either hand activated left dorsal and ventral premotor areas and the supplementary motor cortex regardless of task complexity. The effector-independent activation of left premotor areas was particularly evident in the simple motor imagery task and suggests a left hemispherical dominance for kinesthetic movement representations in right-handed subjects.

The role of the right hemisphere in aphasia recovery after left hemisphere damage remains unclear. Increased activation of the right hemisphere has been observed after left hemisphere damage. This may simply reflect a release from transcallosal inhibition that does not contribute to language functions. Alternatively, the right hemisphere may actively contribute to language functions by supporting disrupted processing in the left hemisphere via interhemispheric connections. To test this hypothesis, we applied off-line continuous theta burst stimulation (cTBS) over the left inferior frontal gyrus (IFG) in healthy volunteers, then used functional MRI to investigate acute changes in effective connectivity between the left and right hemispheres during repetition of auditory and visual words and pseudowords. In separate sessions, we applied cTBS over the left anterior IFG (aIFG) or posterior IFG (pIFG) to test the anatomic specificity of the effects of cTBS on speech processing. Compared with cTBS over the aIFG, cTBS over the pIFG suppressed activity in the left pIFG and increased activity in the right pIFG during pseudoword vs. word repetition in both modalities. This effect was associated with a stronger facilitatory drive from the right pIFG to the left pIFG during pseudoword repetition. Critically, response became faster as the influence of the right pIFG on left pIFG increased, indicating that homologous areas in the right hemisphere actively contribute to language function after a focal left hemisphere lesion. Our findings lend further support to the notion that increased activation of homologous right hemisphere areas supports aphasia recovery after left hemisphere damage.transcranial magnetic stimulation | dynamic causal modeling | virtual lesion | Broca's area N umerous functional imaging studies have reported increased language-related activation of the right inferior frontal gyrus (IFG) in aphasic patients with left hemisphere damage (1-3). However, it is still a matter of debate whether the temporary recruitment of homologous right hemisphere areas after left hemisphere stroke is essential for language performance (i.e., adaptive plasticity) (2-4) or represents "maladaptive" overactivation resulting from interhemispheric disinhibition after left hemisphere infarction (5-7).In the present study, we investigated the adaptive short-term plasticity that supports speech production after disruption to left frontal language areas. We induced neural activity related to phonetic encoding by comparing reading and auditory repetition of pseudowords and familiar words. We expected to see common effects in both visual and auditory modalities at the level of phonetic encoding, but not at the sensory input level (8). We applied transient virtual lesions in healthy volunteers to test whether an up-regulation of right hemisphere homologous language regions after a focal perturbation of left hemisphere language areas reflects reduced transcallosal inhibition from the left hemisphere to the right hemisphere or an active righ...

SUMMARY:There are substantial challenges in the radiologic evaluation of tumor size during clinical trials, and it is important for neuroradiologists to have a firm understanding of these issues. This review will examine measurement approaches, response criteria, selection of lesions for measurement, technical imaging considerations, interval between tumor measurements and response confirmation, and validity of imaging as a measure of efficacy.

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