Event time-locked frequency analyses of EEG allows for the measurement changes in EEG power and phase synchrony, across trials, on a millisecond time scale. In particular, event-related spectral perturbation (ERSP) is a temporally sensitive index of the relative change of mean EEG power from baseline associated with stimulus presentation or response execution. Unlike ERPs, ERSPs capture changes in spontaneous EEG activity that occurs across several frequency spectra and are sensitive to fluctuations that are temporally stable, but not coherent in phase angle (Makeig, 1993; Makeig et al., 2004). Although ERSPs are able to capture induced power changes, which are not revealed in typically averaged ERPs, they do not reveal details about the coherence in phase angle of the event-related EEG signals.
