New approach to localize speech relevant brain areas and hemispheric dominance using spatially filtered magnetoencephalography

Abstract

We used a current localization by spatial filtering-technique to determine primary language areas with magnetoencephalography (MEG) using a silent reading and a silent naming task. In all cases we could localize the sensory speech area (Wernicke) in the posterior part of the left superior temporal gyrus (Brodmann area 22) and the motor speech area (Broca) in the left inferior frontal gyrus (Brodmann area 44). Left hemispheric speech dominance was determined in all cases by a laterality index comparing the current source strength of the activated left side speech areas to their right side homologous. In 12 cases we found early Wernicke and later Broca activation corresponding to the Wernicke-Geschwind model. In three cases, however, we also found early Broca activation indicating that speech-related brain areas need not necessarily be activated sequentially but can also be activated simultaneously. Magnetoencephalography can be a potent tool for functional mapping of speech-related brain areas in individuals, investigating the time-course of brain activation, and identifying the speech dominant hemisphere. This may have implications for presurgical planning in epilepsy and brain tumor patients.

Phonological processing in language production: time course of brain activity

“…First, whereas Thierry et al observed a primacy for Wernicke's over Broca's area, the reversed pattern was found for production in the present study, with the mean temporal delay between the two peaks being identical (B3 s). This result clearly speaks against the hypothesis of a general primacy for Wernicke's area that might have been supposed on the grounds of the reported studies [19,21]. Rather, the present data suggest that the cortical network for phonological processing is identical for production and comprehension with respect to the regions involved but characterised by a variable dynamics.…”

“…Evidence for the fact that the neural activity in Wernicke's area also precedes that in Broca's area during language production was provided by Kober et al [21] by means of a MEG dipole analysis. In two tasks (silent naming, silent reading), the authors observed a temporal primacy for the activity in Wernicke's area.…”

Localization of Broca's area using verb generation tasks in the MEG: Validation against fMRI

“…The development of MEG protocols to study expressive language has been more challenging for a number of reasons, the primary one being the computational limitations of the traditional dipole source localization method [14]. Advances in source localization methods have addressed this issue, and several groups have reported identifying Broca's area in adults [15][16][17][18][19].…”

The Role of MEG in Unveiling Cognition

“…For instance, to shed light on the time course of processing in visual word recognition, visual recognition of heteronymous-phonological and semantically distinct words that share a common orthography, providing a unique opportunity to distinguish between lexical and word form properties, were examined with MEG, finding results that suggest that true lexical access does not occur prior to 200 ms, but rather at a latter stage of processing (Solomyak & Marantz, 2009). A covert name and covert reading task concluded that MEG is a valuable tool to investigate both the time course of speech-evoked cortex activation as well as the localization of the main speech areas (Kober et al, 2001). The N400 component has also been addressed with MEG.…”