Optical imaging of visual cortex epileptic foci and propagation pathways

Precise localization of neocortical epileptic foci is a complex problem that usually requires ictal video-electroencephalography (EEG) recordings; high-resolution magnetic resonance imaging (MRI), positron emission tomography (PET), and single photon emission computed tomography (SPECT) studies; and/or invasive monitoring with implanted grid array electrodes. The exact ictal-onset site must be identified and removed to obtain the best opportunity for a seizure-free outcome. The goal of this study was to determine if high-resolution optical imaging could precisely identify neocortical epileptic foci and what role underlying neuroanatomic pathways played in the seizure propagation. Small acute epileptic foci (0.5 × 0.5 mm(2) ) were created in the primate visual neocortex and single-unit and surface EEG recordings were combined with optical imaging of voltage-sensitive dye changes. Brief visual stimulation was used to evoke interictal bursts. In addition, different visually evoked epileptiform bursts were analyzed to determine the location of the epileptic focus. Spike-triggered averaging of the optical images associated with the surface EEG interictal bursts were analyzed to determine the exact location of the epileptic focus. Specific orientations of brief visual stimulation evoked different intensity optical changes and precisely localized the epileptic focus. Optical imaging identified individual epileptic foci that were <3 mm apart. The development of individual epileptic focus was monitored with optical imaging, which demonstrated excitatory activity at the focus with a surrounding zone of inhibitory-like activity. Propagation pathways outside of the inhibitory-like surround demonstrated alternating bands of excitation and inhibition with a pattern orthogonal to the ocular dominance columns. This experimental study demonstrates that optical imaging can precisely localize an epileptic focus, and provides excellent spatial resolution of the changes that occur in and around the site of the epileptic focus.