Abstracts

Successful epilepsy surgery requires presurgical mapping of eloquent cortex, including cortical language areas. The intracarotid sodium amytal test (IAT) has been the gold standard for presurgical language lateralization. Due to the invasive nature of IAT, there is interest in non-invasive techniques for language lateralization. Functional magnetic resonance imaging (fMRI) allows for reliable lateralization of language function and has replaced IAT in many institutions. However, due to the critical nature of presurgical language mapping, it is desirable to develop adjunct methods to support fMRI .
Magnetoencephalography (MEG) is a non-invasive measure of extracranial magnetic fields produced by neuronal activity. MEG has been proposed as a complementary method for presurgical language lateralization.
This study examines the concordance of fMRI and MEG for language lateralization in children with epilepsy. Functional MRI and MEG were performed for language lateralization in 10 children, aged 8-18 years, being considered for epilepsy surgery. All children were diagnosed with medically refractory epilepsy, with right or left, frontal or temporal epileptic foci documented by video-electroencephalography.
BOLD fMRI was performed at 1.5T using 2 of 4 covert language paradigms. Following post processing of MR data, individual studies were visually inspected for frontal and temporal language areas and rated for laterality.
During MEG, subjects performed an auditory recognition memory paradigm, previously reported to activate receptive language areas. Dipoles were computed for each time point of the sustained polarity reversal following the primary auditory response and mapped on structural images to localize language to left, right or bilateral temporal language areas. Magnetoencephalography and fMRI were concordant for hemisphere dominance in 6 of 10 children. In 4 children, fMRI demonstrated bilateral language activation while MEG identified single hemisphere dominance. Functional MRI and MEG are not consistently concordant for language lateralization in children with epilepsy. Discordance may reflect altered sensitivity to language tasks, differences in the language paradigms used or inherent differences in the underlying substrates of activation. Magnetoencephalography evaluates extracranial magnetic fields as a correlate of brain activity, while fMRI exploits the hemodynamic changes associated with neuronal firing. Furthermore, MEG measures synchronized activations which are time locked to stimuli, while fMRI activation maps represent a summation of the BOLD response compared to a control task.
Further studies are needed to delineate the factors underlying this discordance, including comparative studies of MEG and fMRI using identical language paradigms.