Abstracts

RATIONALE: To report electrocortical mapping results using subdural electrodes in four patients with focal epilepsy being evaluated for epilepsy surgery.
METHODS: All four patients were candidates for occipital lobectomy or basal temporal topectomies. Subdural electrodes were implanted, consisting of grid and/or strip electrodes. An Ojemann Cortical Stimulator (Radionics Inc., Burlington, MA) was used to activate adjacent pairs of electrodes at currents 0.5 to 8 milliamperes for 3 seconds in duration. Fifty-one pairs of electrodes were stimulated with positive responses in 25 pairs. Stimulations producing afterdischarges were not included in the analysis. A 40*70 cm2 cardboard, divided into equal quadrants by perpendicular lines, was held 3o cm from the patient. The patients[ssquote] eyes were fixed at the center of the board during stimulation allowing the patients to indicate the location of the visual response.
RESULTS: Colorful [dsquote]target[dsquote] phosphenes (2 patients), which tended to be centrally located, were activated by stimulation near to the calcarine sulcus. White or unicolor phosphenes (4 patients) tended to be located more peripherally or would move from the center to the periphery, but remained in quadrants opposite to the association cortex stimulated. Formed hallucinations and illusions of spatial rotation were reported by individual patients with stimulation of the lateral temporo-occipital and laterobasal occipital areas, respectively.
CONCLUSIONS: The characterization of phosphenes produced by occipital lobe stimulation may differentiate the primary visual area and peristriate visual association cortices. Formed hallucinations and the illusion of movement were activated in occipitotemporal association areas.