Electrical Stimulation for Seizure Induction and Functional Mapping in Stereoelectroencephalography

The challenge in epilepsy surgery is to remove the epileptogenic zone without compromising postoperative function. Electrical stimulation (ES) was developed to identify with greater anatomical and physiological precision cortical structures essential to language and motor function. Because the language network is widely distributed in the left (dominant) hemisphere, and often varies among patients, brain mapping should delineate eloquent areas at the individual level. Although ES for language is commonly performed during a presurgical investigation, the procedure remains nonstandardized among centers, and differences between grids and intracerebral depth electrodes (stereoelectroencephalography) should be taken into account when comparing results from different series. Electrical simulation exerts a complex summation effect in a given volume of brain tissue; the net result may be difficult to predict because this depends on methodological and physiological factors as follows: stimulation parameters, type of electrodes used, tasks used, presence or not of an after-discharge. We discuss in the present article some methodological aspects of the stimulation procedure influencing functional mapping because it pertains in particular to language mapping. In stereoelectroencephalography, the other goal of ES is to trigger seizures. Because seizures develop in neural networks that are imperfectly sampled by intracerebral electrodes, spontaneous seizure recording is not always sufficient to properly localize the epileptogenic zone. The rationale for performing ES is based on the electrophysiological distinction between local/regional after-discharge and seizure networks. A different response to ES-induced seizures can be seen for supra- and infra-sylvian epilepsies. The procedures for triggering seizures and for functional mapping should be performed in conjunction to answer the fundamental question whether or not there is a spatio-temporal overlap between the epileptogenic and the functional network. The exploration of both networks through stimulation exemplifies the importance of anatomo-electro-clinical correlations in stereoelectroencephalography.