EEC coherency II: experimental comparisons of multiple measures

Objective: A concentric spheres model was used in an earlier paper to estim ate the effects of volume conduction, reference electrode and spatial filte ring on different EEG coherence measures. EEG data are used here to verify theoretical predictions. Methods: Three EEG data sets were: (1) 64 channel, recorded during 7 altern ating periods of resting and mental calculation. (2) 128 channel, for compa rison of eyes open versus eyes closed coherence. (3) 128 channel, recorded during deep sleep (stages 3 and 4) and REM. Results: The directions of large scale (lobeal) coherency changes between b rain states are relatively independent of coherence measure. However, coher ence between specific electrode pairs is sensitive to method and frequency. Average reference and digitally Linked mastoids provide reasonable semi-qu antitative estimates of large-scale neocortical source coherence. Close bip olar, Laplacian, and dura image methods remove most reference electrode and volume conduction distortion, but may underestimate coherence by spatial f iltering. Conclusion: Each EEG coherence method has its own potential sources of erro r and provides coherence estimates for different neural population sizes lo cated in different locations. Thus, studies of coherence and brain state sh ould include several different kinds of estimates to take full advantage of information in recorded signals. (C) 1999 Elsevier Science Ireland Ltd. Al l rights reserved.