Abstracts

Rationale: Anatomic asymmetries in the human temporal lobes, first described in the 1960’s, are believed to be related to language dominance. Several studies have investigated a relationship between structural asymmetries on magnetic resonance imaging (MRI) and language dominance established by the Wada test and functional MRI (fMRI). Individuals who show typical (left) cerebral language dominance tend to have more pronounced leftward asymmetries than those with atypical (right or bilateral) language dominance. To date, no studies have considered the association between anatomic cerebral asymmetries and MSI-based language dominance. Early left hemisphere injury/seizure onset often results in functional reorganization of language dominance to the non-injured hemisphere. It has not yet been determined whether structural adjustments accompany this shift. In the current study, we examined the lengths and angles of the SF in the left and right hemispheres in relation to functional language dominance established by MSI. Methods: The sample consisted of 18 epilepsy surgery candidates who had undergone MSI to assess cerebral dominance for language. Patients were over 18 years of age, had a FSIQ > 65, and had no structural brain abnormality other than mesial temporal sclerosis. Fourteen patients were right-handed; 12 were female. The magnetoencephalography unit consisted of a 148-channel Magnes 2500 WH system housed in a magnetically shielded room. The single equivalent current dipole (ECD) model across each whole hemisphere was used in the analysis. Receptive language was assessed using an auditory word recognition task in which subjects were requested to raise an index finger when they heard designated target words repeated in successive lists. Measurements were performed using the OSIRIX software program, which allowed for viewing and measurement of structures seen on 3-Tessla Magnetic Resonance Imaging scans. The SF was measured on one sagittal slice at approximately 1 centimeter medial to each lateral surface. Results: Eleven of 18 patients were classified left language dominant on MSI. Of the 7 individuals with atypical language dominance on MSI, only 2 showed atypical SF length asymmetry (i.e., no difference or right longer than left), and only 1 showed atypical SF angle asymmetry (i.e., no posterior angle on right). Of the 8 individuals with seizure onset < age 3 years, 3 were classified left language dominant and 5 right language dominant. Most of these individuals showed the expected structural asymmetries typically associated with left language dominance (6 showed the expected length asymmetry and 7 the expected angle asymmetry). Conclusions: In this small sample, atypical functional language organization was not associated with atypical interhemispheric anatomic asymmetries or with age of seizure onset. These data raise the speculation that functional language reorganization may not result in any structural changes within the temporal lobes.