Abstracts

Rationale: Evaluation of verbal memory lateralization is critical for epilepsy surgery planning. The intracarotid amobarbital procedure (IAP), or Wada test, has been a standard technique to determine the memory lateralization, however, it is semi-invasive procedure that has some risks. Magnetoencephalography (MEG) non-invasively detects neuromagnetic signals associated with cognitive tasks. We assessed the usefulness of verbal memory lateralization represented by MEG in patients with epilepsy.Methods: Nine epilepsy patients (male:4, female:5, age:13-34) were studied. Informed consent was obtained from each patient or guardian. MEG was recorded with a 306-channel whole-head system at a sampling rate of 1KHz. In all patients, high-resolution 3T anatomical MRI data were acquired (MPRAGE). A visual word retrieval task was performed. Patients decided whether a word was presented in a word presenting session prior to the retrieval task. All trials were selectively based on whether it was a correct or incorrect, and whether it was a new or old word. We calculated dynamic statistical parametric maps (dSPMs) by applying a distributed source model mapped on the MRI-derived cortical surface. The activation between 400ms and 1000ms after the trigger was analyzed. We obtained the activation value of each unit current dipolar source in the following cortical areas; parsopercularis, parstriangularis, superior temporal, middle temporal, supramarginal, inferior parietal and parahippocampal cortices on both hemispheres, based on a FreeSurfer parcellation. Laterality index of dSPMs (LI-dSPMs) was obtained by LI = (L-R)/(L+R), where L and R is the number of unit dipoles with an activation value higher than the threshold value in these cortical areas of left and right hemisphere, respectively. We also calculated equivalent current dipoles (ECDs) sequentially between 400ms and 1000ms based on a single dipole model by using the left and right hemispheric sensors separately. Laterality index of ECDs (LI-ECDs) was obtained by LI = (L-R)/(L+R), where L and R is the number of ECDs with goodness of fit higher than 60% in the left and right hemisphere. For each patient, memory predominance was determined based on the laterality index as follows; ?0.1:left, 0.1>LI>-0.1:bilateral, ?-0.1:right. We compared the results of LI-dSPMs and LI-ECDs with WADA test. Results: In dSPMs-LI. seven patients showed the memory predominance on the same side as represented in the IAP (four: left, two: right, 1: bilateral). Two patients showed left and right predominance in dSPMs-LI and IAP, and one was bilateral in MEG and right on IAP. In ECDs-LI, the MEG lateralization of six patients was consistent with Wada test (three: left, two: right, one: bilateral). Two patients were left in ECDs-LI and right in IAP. Overall sensitivity was 0.78 for dSPMs-LI and 0.67 for ECDs-LI.Conclusions: Verbal memory lateralization determined by LI-dSPMs and LI-ECDs were consistent with IAP lateralization, suggesting the feasibility of using a MEG for investigating memory lateralization non-invasively.