In addition to the applications described above, which are directly relevant for neurosurgical applications, the MEG is becoming an important tool for the analysis of dynamical brain activities in relation to neuro-cognitive functions (Momjian, Seghier et al. 2003). In this context the functional mapping of speech relevant areas is also clinical useful since in neuro-surgery it is of the uttermost importance to avoid damage to these eloquent areas of the brain. This is usually done by way of the invasive Wada test (Wada and Rasmussen 1960). The question is whether the MEG may be an alternative non-invasive method to the Wada test. A number of studies, particularly of the Helsinki school, have shown that MEG signals evoked by speech related tasks can be recorded and localized to speech relevant brain areas (Fig 3). A picture naming task in normal subjects revealed that MEG fields evoked by the visual presentation of pictures to be named appear first in the occipital areas and evolve in time from the posterior to frontal areas (Salmelin, Hari et al. 1994). These MEG signals excised brain tissue revealed the presence of extensive gliosis secondary to the intra-cerebral drainage performed earlier. The patient has remained seizure free, on anti-epileptic medication, 14 months after surgery. (Adapted with permission from (van't
Ent, Manshanden et al. 2003))
— passive viewing
— picture naming
— passive viewing
— picture naming picture speech
Salmelin et al. Nature 1994
LI. tlOnAm 0 500 ms picture speech
Salmelin et al. Nature 1994
Fig. 3. Brain activation in the course of a naming-aloud experiment (orange traces) of pictures presented on a computer screen once every 5 s. The early visual MEG evoked response (source 1, picture upper left) at the back of the head was followed by more lateral posterior signals in both hemispheres (sources 2 and 3, upper middle and right), suggesting a contribution from Wernicke's area and its right-sided counterpart. Within the same time window (grey rectangles), a left frontal site (4, below left) close to the cortical face representation area showed activation, as in preparation for mouth movements. About 500 ms after the picture had been presented, signals emerged from a site anterior to the face representation area in the motor cortex, reflecting activation of Broca's area (5, below middle) and its counterpart in the right hemisphere. These signals were immediately followed by activity at the top of the brain (6, below right), probably generated in the vicinity of the supplementary motor area. MEG responses during passive viewing (white traces) and quiet naming (green traces) are also shown. The locations of visual (V), auditory (A), and somatosensory (S) projection cortices are marked by yellow squares. (Adapted with permission from (Salmelin, Hari et al.
are bilateral but over the left hemisphere they are usually stronger and leading in time. In a more specific task Gootjes, Raij et al. (1999) performed a MEG study in 11 healthy right-handed subjects to estimate individual hemispheric dominance for speech sounds. The auditory stimuli comprised binaurally presented vowels, tones, and piano notes in groups of two or four stimuli. In the left hemisphere, vowels evoked significantly stronger (37-79%) responses than notes and tones, whereas in the right hemisphere the responses to different stimuli did not differ significantly. Specifically, in the two-stimulus task, all 11 subjects showed left-hemisphere dominance in the vowel vs tone comparison. This simple paradigm may be helpful in non-invasive evaluation of language lateralization. Other MEG studies using different forms of signal analysis are congruent with these findings. Papanicolaou et al. (1999) reported that they could identify the dominant hemisphere by inter-hemispheric comparison of the number of dipolar sources, and the same applies to the study of Kober (2001) who used a laterality index calculated from the strength of current density distributions after spatial filtering.
More well controlled studies in larger groups of subjects are, of course, necessary. Nevertheless the results obtained until now allow us to conclude that MEG is a valuable method to investigate the dynamics of speech processing. Furthermore MEG merits being explored as a potential alternative to the Wada test for non-invasive presurgical localization of speech eloquent brain areas and the identification of the language dominant hemisphere.
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