P243 Functional connectivity analysis of cortico-cortical evoked potentials

Objectives The electrical stimulation of the cortical tissue can evoke early (N1) and late (N2) cortico-cortical evoked potential (CCEP) components. In this study, we applied functional connectivity analysis to investigate the attenuation and network topology induced by electrical stimulation. We also compared the synchronization and network topology in case of stimulations inside and outside the epileptic zone (EZ). Methods 16 presurgery epileptic patients were analyzed with subdural grid positions. The CCEP mapping was performed by injecting current pulses (10 mA, 0.2 ms pulse width, 25 trials) into all adjacent electrode contacts. Phase synchronization across trials was applied on the N1 (10–50 ms; filtered to 10–40 Hz) and N2 (50–500 ms; filtered to 1–4 Hz) evoked potentials to measure functional connectivity. We applied Minimum Spanning Tree analysis to express topological parameters of the networks. Results N1 had higher local synchrony than N2 and N1 had faster attenuation from the stimulation point toward more distant electrodes than N2. Furthermore, N1 was characterized by a more star-like network topology than N2. N1 synchrony was higher and the network became more centralized when the current was injected to the EZ. Conclusion The neural mechanism of N1 is based on direct cortico-cortical connections while N2 is thought to be the result of more complex processes. This can explain our more distributed N2 network compared to the local N1 network. Significance The extensive synchrony and centralized topology of the N1 network in case of EZ stimulation supports the increased excitability of the epileptogenic zone.