Abstracts

Rationale: Cortico-cortical evoked potentials (CCEP) in surgical epilepsy patients are increasingly used to study the basic neuroscience of functional networks as well as pathological networks in epilepsy research. An increasing amount of CCEP studies are becoming available but often, different stimulation parameters are used across studies. Also, within single subjects networks are obtained by stimulating different cortical and subcortical areas with the same stimulation parameters. However, to date little is known about the effect of stimulation parameters such as amplitude, frequency, or location of stimulation on the amplitudes and network topologies of CCEPs. This knowledge is crucial for the interpretation of CCEP studies. Methods: CCEPs were recorded in four intractable epilepsy patients undergoing intracranial EEG monitoring for presurgical evaluation. Single pulse stimulation (100us/phase, 20 trials) was applied to every adjacent electrode pair on the implanted grids and strips and the evoked potentials (CCEPs) were recorded at each remaining electrode. Four different stimulation amplitudes were used (4, 6, 8, 10 mAmp) and the stimulation frequency was varied between .5 to 1Hz. The underlying cortical area of each electrode was automatically identified by co-registering the electrodes to the patient's surface parcellation (Freesurfer). The effect of the various stimulations was compared across different cortical and subcortical sites. Evoked responses were quantified in early (10-50ms) and late (50 - 500ms) windows. Results: As expected, a positive relationship between increasing stimulation amplitudes and CCEP peak responses were observed. No clear saturation within the used stimulation amplitudes was seen in the early window but seemingly in the late window. The topology of networks was overall stable but showed some modulation by stimulation amplitudes. Conclusions: Knowing the effects of stimulation parameters on CCEPs is crucial for being able to infer functional and pathological network connectivity from CCEPs. Our results show a robust effect of stimulation amplitude and location on the evoked responses. These results should be taken into account when reading available studies and will help guide the planning of future CCEP experiments.