Graph Theoretical Characteristics of EEG-Based Functional Brain Networks in Patients With Epilepsy: The Effect of Reference Choice and Volume Conduction

Abstract

<p> It is well-established that both volume conduction and the choice of recording reference</p> <p>(montage) affect the correlation measures obtained from scalp EEG, both in the time</p> <p>and frequency domains. As a result, a number of correlation measures have been</p> <p>proposed aiming to reduce these effects. In our previous work, we have showed that</p> <p>scalp-EEG based functional brain networks in patients with epilepsy exhibit clear periodic</p> <p>patterns at different time scales and that these patterns are strongly correlated to seizure</p> <p>onset, particularly at shorter time scales (around 3 and 5 h), which has important clinical</p> <p>implications. In the present work, we use the same long-duration clinical scalp EEG data</p> <p>(multiple days) to investigate the extent to which the aforementioned results are affected</p> <p>by the choice of reference choice and correlation measure, by considering several widely</p> <p>used montages as well as correlation metrics that are differentially sensitive to the</p> <p>effects of volume conduction. Specifically, we compare two standard and commonly</p> <p>used linear correlation measures, cross-correlation in the time domain, and coherence in</p> <p>the frequency domain, with measures that account for zero-lag correlations: corrected</p> <p>cross-correlation, imaginary coherence, phase lag index, and weighted phase lag index.</p> <p>We show that the graphs constructed with corrected cross-correlation and WPLI are</p> <p>more stable across different choices of reference. Also, we demonstrate that all the</p> <p>examined correlation measures revealed similar periodic patterns in the obtained graph</p> <p>measures when the bipolar and common reference (Cz) montage were used. This</p> <p>includes circadian-related periodicities (e.g., a clear increase in connectivity during sleep</p> <p>periods as compared to awake periods), as well as periodicities at shorter time scales</p> <p>(around 3 and 5 h). On the other hand, these results were affected to a large degree when</p> <p>the average referencemontage was used in combination with standard cross-correlation,</p> <p>coherence, imaginary coherence, and PLI, which is likely due to the low number of</p> <p>electrodes and inadequate electrode coverage of the scalp. Finally, we demonstrate that</p> <p>the correlation between seizure onset and the brain network periodicities is preserved </p> <p> when corrected cross-correlation and WPLI were used for all the examined montages.</p> <p>This suggests that, even in the standard clinical setting of EEG recording in epilepsy</p> <p>where only a limited number of scalp EEG measurements are available, graph-theoretic</p> <p>quantification of periodic patterns using appropriate montage, and correlation measures</p> <p>corrected for volume conduction provides useful insights into seizure onset.</p>