Measuring phase synchrony in brain signals

This article presents, for the first time, a practical method for the direc t quantification of frequency-specific synchronization (i.e., transient pha se-locking) between two neuroelectric signals. The motivation for its devel opment is to be able to examine the role of neural synchronies as a putativ e mechanism for long-range neural integration during cognitive tasks. The m ethod, called phase-locking statistics (PLS), measures the significance of the phase covariance between two signals with a reasonable time-resolution (<100 ms). Unlike the more traditional method of spectral coherence, PLS se parates the phase and amplitude components and can be directly interpreted in the framework of neural integration. To validate synchrony values agains t background fluctuations, PLS uses surrogate data and thus makes no a prio ri assumptions on the nature of the experimental data. We also apply PLS to investigate intracortical recordings from an epileptic patient performing a visual discrimination task. We find large-scale synchronies in the gamma band (45 Hz), e.g., between hippocampus and frontal gyrus, and local synchr onies, within a Limbic region, a few cm apart. We argue that whereas long-s cale effects do reflect cognitive processing, short-scale synchronies are L ikely to be due to volume conduction. We discuss ways to separate such cond uction effects from true signal synchrony. (C) 1999 Wiley-Liss, Inc.