Magnetocephalography: a noninvasive alternative to the Wada procedure

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Object. In this study the authors evaluated the sensitivity and selectivity of a noninvasive language mapping procedure based on magnetoencephalography (MEG), for determining hemispheric dominance for language functions.

Methods. Magnetic activation profiles of the brain were obtained from 100 surgical candidates (age range 8–56 years) with medically intractable seizure disorder by using a whole-head MEG system within the context of a word recognition task. The degree of language-specific activity was indexed according to the number of consecutive sources (modeled as single, moving current dipoles) in perisylvian brain areas. Only activity sources that were observed with a high degree of spatial and temporal overlap in two split-half data sets were used to compute the MEG laterality index. Independently, all patients underwent Wada testing for the determination of hemispheric dominance for language.

Independent clinical judgments based on MEG and Wada data showed a high degree of concordance (87%). Magnetoencephalography laterality judgments had an overall sensitivity of 98%, but a lower selectivity of 83%, which was due to the fact that MEG detected more activity in the nondominant hemisphere than was predicted based on the Wada test. A number of objective criteria were derived based on this large patient series to ensure data quality and bolster the clinical usefulness of MEG for language mapping.

Conclusions. Although the availability of MEG is still limited across epilepsy surgery centers, this study method may be substituted for the Wada procedure in assessing hemispheric dominance for language in select cases.

Article Information

Address reprint requests to: Andrew C. Papanicolaou, Ph.D., Division of Clinical Neurosciences, Department of Neurosurgery, University of Texas Health Sciences Center at Houston, 1333 Moursund Street, Suite H114, Houston, Texas 77030. email: Andrew.C.Papanicolaou@uth.tmc.edu.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Upper: Tracing (left) of averaged ERF waveforms recorded from the entire set of 148 magnetometer sensors during a typical MEG recording session. The inset (right) displays the isofield contour map of magnetic flux recorded at the peak of the auditory N1m response. Notice the symmetry between the left and right hemisphere maps. Center: Tracing (left) of an example of unacceptable MEG data due to excessive contamination by large-amplitude rhythmic background activity (in this case primarily in the alpha band, originating from occipital cortices indicated in the inset [right]). Lower: Tracing (left) of an example of data sets not used for clinical judgment due to the presence of a significant lateral asymmetry in the amplitude of the N1m response (that is, |[left N1m RMS − right N1m RMS]/[left N1m RMS + right N1m RMS]|) > 0.3).

  • View in gallery

    Typical activation profiles obtained in patients with left- (upper) or right-hemisphere dominance (lower), or bihemispheric representation of language function (center). Activity sources located in the left hemisphere are featured on the left side of the figure and those computed in the right hemisphere are featured on the right. Each activation profile represents sources computed after initial sensory activation (> 200 msec poststimulus onset) and were actually localized within 1.5 cm from the sagittal slice selected for display. Activity sources obtained during the first half of the activation trials are shown as green circles, whereas corresponding sources obtained during the second half are shown as red circles. The region of spatial overlap between the two maps within a given hemisphere (lataral projection) is marked by a white circle. This region has been shown in previous studies to correspond with the receptive language—specific cortex in patients who, in addition to the Wada procedure, underwent electrocortical stimulation mapping within the dominant hemisphere.

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