Observation of unaveraged giant MEG activity from language areas during speech tasks in patients harboring brain lesions very close to essential language areas: expression of brain plasticity in language processing networks?

We describe a series of 12 patients who suffered from lesions adjacent to the classic Broca and Wernicke areas and were examined by magnetoencephalography (MEG) for presurgical language localization while performing a protocol of different language tasks. In these patients very large MEG activity of up to 5pT was observed, which was located not only in the adjacent language processing brain areas but also in more distant areas, which are part of the language processing neuronal network. The high amplitude and the focal spatial extent of this activity allowed MEG source localization from the unaveraged data. In nine patients sources of this high amplitude activity were even found in the homologous language areas on the contralateral, the nondominant side of the brain. The physiological interrelationship of these large MEG changes needs to be investigated in more detail in further studies especially in the context of possible mechanisms for brain plasticity to overcome inhibitory activity of the impaired language area.

The electrical and magnetical cerebral responses evoked by electrical stimulation of the esophagus and the location of their cerebral sources

OBJECTIVES

After electrical stimulation of the esophagus cerebral responses are recordable, their cortical source is under discussion. Brain mapping using electroencephalography recordings demonstrated partially controversial results. Sources of evoked responses can be localized more easily using magnetoencephalography than electroencephalography.

METHODS

We examined 22 volunteers by recording electrical somatosensory potentials after electrical stimulation of the esophagus. In 9 of these 22 subjects additional recording of magnetic fields was performed and the sources of the evoked magnetic fields were computed.

RESULTS

The evoked potentials after electrical stimulation of the esophagus had a similar latency as the previously published data. The source localization done by magnetoencephalography suggest that first a region of the postcentral gyrus is activated which is temporo-lateral to the primary somatosensory cortex of the pharynx. This region is suggested to be the primary somatosensory region of the esophagus. This source was followed by a source in the parietal operculum thought being part of the secondary somatosensory cortex. Simultaneously the insular cortex was activated pointing to a parallel neuronal pathway to the central autonomic nervous system.

CONCLUSION

After electrical stimulation of the esophagus somatosensory cortical areas of the temporal postcentral gyrus and the operculum are activated. In parallel activation of the insular cortex as part of the central autonomic network was found.

Responses to silent Kanji reading of the native Japanese and German in task subtraction magnetoencephalography

The neuromagnetic activities evoked by semantic processing were localized by magnetoencephalography (MEG). We observed distinct time courses of the activities in native speaking Japanese subjects (Japanese speaker) and German subjects (German speaker) during silent reading of Japanese letters; Kanji and meaningless figures made by deforming the Arabian letters. There were significant differences in amplitude of the activities between Kanji and meaningless figure stimuli. The responses with meaningless figure stimuli were subtracted from those with Kanji stimuli to demonstrate the semantic responses. Earlier responses peaked at about 273.3+/-50. 8 and 245.0+/-23.8 ms (mean+/-S.D.) and were mainly located in the right fusiform gyrus (FuG) in the Japanese and German speakers, respectively. All the Japanese speakers constantly showed additional later responses in the left superior temporal gyrus (STG) and the supramarginal gyrus (SmG) at approximately 616.1+/-105.5 ms, whereas no further activity was observed in the German speakers who did not know the meaning of each Kanji. Because the later responses in the STG and SmG in the Japanese speakers were only observed in their dominant hemisphere, we believe the source of these responses to be part of the neural basis of Kanji semantic processing. The task subtraction MEG analysis could be a powerful method to discriminate distinct responses and visualize the neural networks involved in semantic processing.

Exactness of source analysis of biomagnetic signals of epileptiform spikes by the method of spatial filtering: a computer simulation

On the basis of spatial covariance it is found that, by spatial filtering the localisation of a single dipole source, both parallel and perpendicular to the measurement plane (assuming a signal-noise ratio of 5:1), can be performed with an accuracy of < 0.5 mm. When the signal-noise ratio is increased to 30:1, the resolution of temporally independent current sources separated by 2 mm becomes practicable. This resolution study is carried out by means of a pair of unity current dipoles with the dipole distance as a varying source model parameter. The conclusions, drawn from the results of computer simulation and supported by statistical calculations, refer to the spherical model of the volume conductor of the brain.