Influence of paroxysmal activity on background synchronization in epileptic recordings.

BACKGROUND The presence of spikes and sharp waves in recordings of epileptic patients contaminates background signal synchronization. When estimating functional connectivity between extended cortical areas, the influence of epileptic spikes in specific areas should be considered; however, this step is sometimes overlooked. We present a simple method for quantifying the influence of epileptic activity on background signal synchronization. METHOD Standard synchronization measures were calculated for both pure correlated Gaussian signals and correlated Gaussian signals with different levels of epileptic spikes in order to determine the influence of epileptic activity on synchronization estimates. RESULTS Synchronization from invasive epileptic recordings (e.g., depth electrodes) displays a much higher bias due to epileptic activity than superficial electrodes. Moreover, statistical methods such as mutual information are more affected by spike presence than phase synchronization methods. The influence of spikes is far greater at low values of background synchronization. CONCLUSIONS The information provided by this procedure makes it possible to differentiate true background synchronization from spike synchronization. Thus, our procedure serves as a guide for analyzing synchronization and functional connectivity calculations in epileptic recordings.