The synchronization of neuronal assemblies has been widely studied using the following three measures: correlation (such as cross correlation), phase synchrony (such as mean phase coherence), and coherence [such as magnitude squared coherence (MSC)]. Cross correlation measures the linear correlation between two signals in the time domain [21]. Phase synchrony is a direct index of neural synchrony and is defined by a phase locking value, ranging from zero (no synchronization) to one (perfect synchronization). Coherence identifies the synchrony of neuronal assemblies as a function of the correlation of EEG frequency components. For iEEG, the neural generators are the local assemblies of coherent neurons with common spectral properties. Each of these measures have been used extensively to assess neural synchrony in human electrophysiological studies of brain function and disease. The typical finding is that, in a given pathological state or in a perceptual, cognitive, or motor task, the EEG correlation, coherence, or synchrony increases (or decreases) [7], [13], [19], [22]–[31].
