Abstracts

Rationale: Recent studies have suggested that cortical gamma oscillations are tightly linked with various forms of physiological activity. In the present study, the dynamic changes of intracranially-recorded median nerve SEPs and sensory-evoked gamma oscillations were animated on a 3D-MRI in children being evaluated for epilepsy surgery. We determined whether recordings of SEPs and sensory-evoked gamma oscillations could localize the anatomically-defined post- and pre-central gyri.Methods: A total of 8 children with intractable focal epilepsy who underwent a two-stage epilepsy surgery were studied (age: 3 – 16 years; 5 girls and 3 boys). All patients underwent preoperative MRI and extraoperative electrocorticography (ECoG) recording. SEPs and sensory-evoked gamma oscillations were simultaneously recorded on chronically-implanted subdural electrodes. The median nerve contralateral to the presumed epileptic focus was electrically stimulated resulting in thenar muscle twitching; an averaged SEP was yielded for each electrode, and the electrode showing the earliest N20 peak was identified for each patient. Subsequently, an “N20 Magnitude” (defined as the area under the averaged SEP between 2 ms before and after the earliest N20 peak across all electrodes) was determined for each electrode. In addition, each sensory-evoked trace was transformed into a time-frequency matrix, and a “Sensory-Evoked Gamma Magnitude” (defined as the degree of alteration of amplitude power of 100-250 Hz gamma oscillations compared to the baseline amplitude power just before median nerve stimulation) was sequentially delineated on each individual 3D-MRI. The correlation between “N20 Magnitude” and “Sensory-Evoked Gamma Magnitude” within each patient was determined using Spearman’s rank correlation. It was also determined whether the cortex showing the largest “N20 magnitude” as well as the cortex showing the largest “Sensory-Evoked Gamma Magnitude” at 15 ms was localized to the post- and pre-central gyri. Results: Visual assessment revealed that the dynamic changes of median nerve SEPs and sensory-evoked gamma oscillations mostly involved the post- and pre-central gyri in all children. There was a significant positive correlation between measures of “N20 Magnitude” and “Sensory-Evoked Gamma Magnitude” at 15 ms (mean rho = 0.70; p < 0.01). The largest “N20 Magnitude” was noted in the postcentral gyrus in 5 children and on an electrode overlying the central sulcus in three children. The largest “Sensory-Evoked Gamma Magnitude” was noted in the postcentral gyrus in 7 children and on an electrode overlying the central sulcus in the remaining child. Conclusions: The dynamic changes of SEPs and sensory-evoked gamma oscillations involving both pre- and post-central gyri may be associated with external sensory stimuli accompanied by passive movement of the thumb. Both intracranial recordings of SEPs and sensory-evoked gamma oscillations may be utilized to localize the primary sensory-motor hand area in children with focal epilepsy. Supported by NIH Grant NS47550 (to E.A)