Magnetic source imaging of late evoked field responses to vowels: toward an assessment of hemispheric dominance for language.

Abstract

The goal of this study was to determine whether the late neuromagnetic field elicited by simple speech sounds, which is detected by magnetoencephalography, may be used to estimate hemispheric dominance for language and to guide or constrain the intraoperative search for essential language sites. If sufficiently robust, a noninvasive method for assessing hemispheric dominance for language could reduce the necessity for amobarbital testing and the extent of intraoperative cortical stimulation-based mapping, both of which carry the risk of morbidity. Fifteen patients undergoing surgery for tumors during which intraoperative language mapping would be performed and two additional patients in whom intracarotid amobarbital testing confirmed right-hemisphere language dominance participated. Following a primary auditory response sources of late neuromagnetic fields elicited by vowel stimuli were modeled and coregistered using magnetic resonance images to form magnetic source (MS) images. A laterality index (LI) was calculated by summing the number of equivalent current dipolar sources in the late fields detected from each hemisphere. In 14 right-handed patients, 10 displayed left asymmetric LIs (0.37 +/- 0.16. mean +/- standard error of the mean in 14 patients). For both right-hemisphere dominant patients in whom an LI was obtainable, the LI was rightward. Stimulation-mapped essential language sites were found in 7 of 15 patients. For six of these seven patients, the MS image-derived LI was leftward. Asymmetry in single equivalent dipole modeling of the late neuromagnetic field evoked by simple speech sounds correlates with hemispheric language dominance, although not to the degree necessary for individual clinical predictions. With further development, MS imaging of simple language tasks may be used preoperatively to predict language dominance and even to identify or constrain the intraoperative search for likely sites of essential language cortex.