Indexes that evaluate FC based on signal synchronization and coupling

Connectivity indexDescription
Coherence (Coh)A measure of the covariance of the frequency components of two signals. In EEG studies, COh typically corresponds to the covariance of spectral activity between two electrode locations.
Imaginary part of coherency (iCOh)Coherency is the imaginary part of the Fourier-transformed coherency and measures the linear relationship between two EEG signals at a specific frequency. It shows how the phases of signals in channels i and j are linked, assuming the signals are stable over time.
Phase locking value (PLV)PLV measures how consistent the phase differences are between two brain regions. When PLV is close to 1, it means the regions are highly synchronized. If PLV is close to zero, it indicates large variability in phase differences.
Phase lag index (PLI)PLI measures the asymmetry in the phase difference between two EEG signals. It shows how consistently one signal leads or lags behind the other.
Synchronization likelihood (SL)This measure calculates the likelihood that two signals are in the same “dynamical state”. These states are defined by the time-delay embedded vectors of the signals.
Phase amplitude coupling (PAC)This measure examines the relationship between the phase of a low-frequency oscillation and the amplitude of a higher-frequency oscillation, either within the same brain region or between different regions (e.g., the low-frequency phase in region A influencing the high-frequency amplitude in region B).
Amplitude envelope correlation (AEC)AEC measures the correlation between the amplitude envelopes of two signals at each frequency. It uses Pearson’s r to compare the log-transformed power envelopes of the signals.

EEG: electroencephalography