Scalp EEG Continuous Space ERD/ERS Quantification

In the scope of EEG applications such as Brain Computer Interfaces (BCI) or the evaluation of epileptic activity, the detection of event-related potentials (ERP) and associated event-related desynchronization / synchronization (ERD/ERS) are common goals. The most commonly used method for assessing ERD/ERS consists on the evaluation of EEG power changes upon the event onset in relation to the baseline, reflecting increased/decreased synchronization. Phase synchronization measures have also been used for this purpose, both across event trials as well as between pairs of electrodes. Here, we propose a 2D spatially continuous extension of the Phase Locking Factor (PLF) metric for ERD/ERS quantification, called PLF Field (PLFF), based on measuring spatial variations of the EEG phase. A continuous phase map is estimated from the discrete set of EEG traces by using the Hilbert transform in the analytical signals framework. The synchronization at each arbitrary spatial location on the scalp space is then computed from the magnitude of the phase gradient at that location. The method is illustrated with EEG data obtained in two motor tasks paradigms. Our results indicate that the proposed approach is largely consistent with the conventional power-based ERD/ERS method, but may provide additional information in studies of neuronal synchronization using EEG.