Phase lag index: assessment of functional connectivity from multi channel EEG and MEG with diminished bias from common sources

Abstract

Objective: To address the problem of volume conduction and active reference electrodes in the assessment of functional connectivity, we propose a novel measure to quantify phase synchronization, the phase lag index (PLI), and compare its performance to the well-known phase coherence (PC), and to the imaginary component of coherency (IC).

Methods: The PLI is a measure of the asymmetry of the distribution of phase differences between two signals. The performance of PLI, PC, and IC was examined in (i) a model of 64 globally coupled oscillators, (ii) an EEG with an absence seizure, (iii) an EEG data set of 15 Alzheimer patients and 13 control subjects, and (iv) two MEG data sets.

Results: PLI and PC were more sensitive than IC to increasing levels of true synchronization in the model. PC and IC were influenced stronger than PLI by spurious correlations because of common sources. All measures detected changes in synchronization during the absence seizure. In contrast to PC, PLI and IC were barely changed by the choice of different montages. PLI and IC were superior to PC in detecting changes in beta band connectivity in AD patients. Finally, PLI and IC revealed a different spatial pattern of functional connectivity in MEG data than PC.

Conclusion: The PLI performed at least as well as the PC in detecting true changes in synchronization in model and real data but, at the same token and like-wise the IC, it was much less affected by the influence of common sources and active reference electrodes.

Copyright 2007 Wiley-Liss, Inc.

Figures

Figure 1

Mean phase coherence (PC, averaged over all possible pairs of 64 modeled EEG channels) as a function of coupling strength K in the Kuramoto model with 64 oscillators as a function of overlap between subsequent EEG channels (CS: common sources, ranging from 0 to 16). All results are the average to 10 trials. The first 5,000 samples of each trial were ignored. Epoch length for each trial was 4,096 samples. Mean frequency of the oscillators in the model was 10 Hz, the width of the Lorentz distribution γ = 1. Sample frequency was 500 Hz. These parameters yield a critical value of Kcrit = 2.

Figure 2

Mean phase lag index (PLI, averaged over all possible pairs of 64 modeled EEG channels) as a function of coupling strength K in the Kuramoto model. Parameters are identical with Figure 1.

Figure 3

Mean absolute value of the imaginary part of coherency (IC, averaged over all possible pairs of 64 modeled EEG channels) as a function of coupling strength K in the Kuramoto model. For parameters see Figure 1.

Figure 4

Detail of EEG recording with absence seizure consisting of ∼3 Hz generalized spike‐and‐slow wave discharges. Average reference, filter settings: high pass 0.5 Hz and low pass 48 Hz. Vertical blue bars indicate 1 s intervals. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 5

Mean PC (averaged over all pairs of 21 channels) for different montages (average, source, mastoids, bipolar, and Cz). Each epoch has a length of 4,096 samples (8.18 s). Epoch no. 6 and 7 correspond to the seizure.

Figure 6

Mean PLI (averaged over all pairs of 21 channels) for different montages (average, source, mastoids, bipolar, and Cz). Each epoch has a length of 4,096 samples (8.18 s). Epoch no. 6 and 7 correspond to the seizure.

Figure 7

Mean IC (averaged over all pairs of 21 channels) for different montages (average, source, mastoids, bipolar, and Cz). Each epoch has a length of 4,096 samples (8.18 s). Epoch no. 6 and 7 correspond to the seizure.

Figure 8

Mean PC for 15 subjects with Alzheimer's disease and 13 control subjects with subjective memory complaints. Error bars indicate standard deviations. Results are the average of four epochs (average reference, epoch length 4,096 samples, 21 channels, digitally filtered between 13 and 30 Hz, sample frequency 500 Hz). Total: average of all pairs of 21 channels; intra_s: average of all short, intrahemispheric electrode pairs; intra_l: average of all long intrahemispheric electrode pairs; inter_s: average of all short interhemispheric electrode pairs; inter_l: average of all long interhemispheric electrode pairs. Details of the specific electrode pairs making up the four sub averages can be found in the methods section.

Figure 9

Mean PLI for 15 subjects with Alzheimer's disease and 13 control subjects with subjective memory complaints; cf. Figure 8.

Figure 10

Mean IC for 15 subjects with Alzheimer's disease and 13 control subjects with subjective memory complaints; cf. Figure 8.

Figure 11

Illustration of the spatial distribution of the strongest correlations between pairs of MEG channels using either phase coherence (PC, left column), the phase lag index (PLI, middle column) or the imaginary part of coherency (IC, right column). Data are collected from two different healthy subjects (upper row and lower row). In all maps, only correlations above threshold are displayed. The threshold was chosen such that sufficiently many connections were visible to allow for a proper evaluation of the spatial pattern of supra threshold connections. Eyes closed, no task MEG (sample frequency 312.5 Hz; filter settings: 8–13 Hz). Epoch length 4,096 samples (13.083 s).