Abstracts

Rationale: Epilepsy is a dynamic process involving complex interactions between excitatory and inhibitory neuronal activity during initiation, propagation and termination. Most research to date has involved electrical recordings which can only sample limited regions of cortex. We used voltage sensitive dyes to record seizures, electrical stimulation and interictal spikes in in vivo rat brain to examine the spatiotemporal evolution of individual deflection of the EEG during an epileptic seizure.Methods: The cortex was stained with the new blue dye RH-1692. 1.5μl of 4-aminopyridine (4-AP) was injected into the superficial layer of the neocortex to induce seizures and interictal spikes. A second glass electrode (2-4 MΩ) filled with 0.9% saline was positioned < 1 mm from the 4-AP injection site to record the local field potential (LFP). A tandem lens setup, with one 80mm and one 35mm lenses, was used for imaging. The VSD data was recorded with the MACAM ULTIMA fast camera system with 1 ms temporal resolution and 50 μm spatial resolution. Images were taken for 20 seconds per each session.Results: Seizures activity initiated with low-amplitude high frequency activity (LAHF) and evolved into spike-an-waves (SW) with the frequency of 10-20 Hz. Most deflection of the LAHF and SW events were maximal at the focal point of seizure initiation. Nevertheless, each deflection of the EEG involved a wave that passed over the cortex. Wave propagation was not homogeneous and passed over islands of active cortex while sparing other areas. Interictal spikes propagated to the similar cortex as those of SWs. Cortical stimulation, on the other hand, involved the entire cortex homogeneously. Conclusions: Our data indicate that focal seizures are made up of individual waves of activity involving larger areas of cortex that spread inhomogenously far beyond the site of initiation. Cortical stimulation, on the other hand depolarize the cortex in a rapid, homogeneous fashion.