Abstracts

RATIONALE:_ Electrophysiological mapping of epileptic foci has been the gold standard, both for animal research and human surgical resections despite significant sampling limitations inherent in the technique. We used optical imaging of intrinsic signals, which can record the optical signals associated with neuronal activity from large cortical areas with sub-millimeter resolution, to generate maps of spontaneously active interictal, ictal and secondary mirror foci. METHODS:_Optical reflectance was recorded with a cooled CCD camera at 707nm in visual and somatosensory cortex of 10 adult anesthetized ferrets. Intracortical iontophoresis of bicuculline and 4- aminopyridine was used to generate interictal and ictal foci. Optical data was correlated with field potential and extra-cellular single unit recordings. RESULTS:_Interictal foci were circular with a sharp border and a radius of 2.84( 1.59) mm2. Each spike elicited a focal increase in blood flow and metabolism that generated an optical signal with a 0.3-5% change in reflectance. A large region of a negative optical signal, correlating well with electrophysiologically recorded inhibition, was seen in the surrounding cortex. Ictal onset zones were localized to regions as small as 1-2 mm2 and when non-propagating, were surrounded by a strong inverted optical signal, most likely reflecting inhibitory control mechanisms. Ictal events elicited huge reflectance changes ranging from 30-60%. The optical signal from mirror foci was well localized but had a smaller amplitude (0.4%), area and a delayed onset. CONCLUSIONS:_Optical epilepsy maps can be generated within a matter of seconds with spike-triggered image division, making it a potentially very useful tool for guiding human epilepsy surgery. Epilepsy Foundation of America, American Association of Neurological Surgeons, Von Humboldt Stiftung, Max-Planck Gesellschaft