Abstracts

Rationale: Cortical activation associated with visual tasks can be studied using intracranially-recorded gamma oscillations in patients being evaluated for epilepsy surgery. The goal of this study was to determine if different visual tasks induced differential cortical activations and whether visual-evoked gamma oscillations were altered by interictal spike discharges. Methods: A total of 8 children (age: 3 – 16 years) with medically-uncontrolled seizures who underwent a two-stage epilepsy surgery were studied. All patients underwent preoperative MRI, extraoperative electrocorticography recording, and functional cortical mapping using electrical stimulation as well as visual-evoked gamma oscillations. All patients were given two types of visual tasks: (i) full-field stroboscopic flash stimuli and (ii) central-field picture stimuli consisting of face, house and abstract shapes. The magnitudes of visual-evoked gamma oscillations (60 – 120 Hz) were sequentially delineated and displayed on the patients’ cortical surface reconstructed from 3D MRI. Results: Full-field stroboscopic flash stimuli resulted in increased gamma oscillations in the medial occipital region starting at 30 - 50 ms after stimulus presentation in all children. This was immediately followed by increased gamma oscillations in the lateral occipital region in 6 children; subsequently, increased gamma oscillations were noted in the inferior-posterior temporal region in 2 children. None of the patients showed a significant increase in gamma oscillations in the frontal region. Central-field picture stimuli resulted in sustained increase of gamma oscillations in the lateral occipital region starting at 50 – 120 ms after the presentation in all children. This was immediately followed by sustained increase in gamma oscillations in the inferior-posterior temporal region in all children; subsequently, increased gamma oscillations were noted in the posterior inferior frontal region in 2 children. None of the patients showed a significant increase of gamma oscillations in the medial occipital region. Electrical stimulation of the lateral occipital region resulted in visual symptoms in the central field, whereas electrical stimulation of the medial occipital region resulted in visual symptoms in the peripheral field. In a patient with the syndrome of continuous spike-wave during slow sleep, a substantial proportion of the visual-evoked gamma oscillations were temporarily obscured by interictal spike-wave discharges. Conclusions: Full-field flash stimuli preferentially activate the peripheral visual field with less activation of the visual association cortex. Central-field picture stimuli preferentially activate the central visual field as well as the visual association cortex. Activation of the posterior-inferior frontal region by central-field picture stimuli may be associated with visual searching. The observation that interictal spike discharges obscured visual-evoked gamma oscillations may support the notion that interictal spike discharges temporarily disturb cognitive function in some children with epilepsy. NIH NS47550 (E.A)