[Eeglablist] Reference electrode and coherence

[Slater, Jeremy D]

The short answer to your question is yes. The issue of reference and coherence was nicely addressed in an article by Nunez in the late 90's, and more recently by Hu et el. The primary concern is that one can see artifacts of high or low coherence depending on the reference chosen. No one has come up with a perfect solution for this yet - and remarkably in many papers (including Nolte's original paper arguing for use of the imaginary component), a reference is chosen (in that paper, the right earlobe), but the potential impact of the reference choice on the results is never discussed. Good idea to be aware of it - best way is probably to analyze the data repetitively using different references then determining what if any changes occur in your results.

Nunez, P. L., Srinivasan, R., Westdorp, A. F., Wijesinghe, R. S., Tucker, D. M., Silberstein, R. B., & Cadusch, P. J. (1997). EEG coherency: I: statistics, reference electrode, volume conduction, Laplacians, cortical imaging, and interpretation at multiple scales. Electroencephalography and clinical neurophysiology, 103(5), 499-515.

Hu, S., Stead, M., Dai, Q., & Worrell, G. A. (2010). On the recording reference contribution to EEG correlation, phase synchorony, and coherence. Systems, Man, and Cybernetics, Part B: Cybernetics, IEEE Transactions on, 40(5), 1294-1304.

JDS

Jeremy D. Slater, MD
Professor of Neurology
Kraft W. Eidman Development Board Professor in the Medical Sciences
UT Health
MHH EMU and Clinical Neurophysiology Laboratory Director
Director, Texas Comprehensive Epilepsy Program
Mischer Neuroscience Institute
________________________________
From: eeglablist-bounces at sccn.ucsd.edu [eeglablist-bounces at sccn.ucsd.edu] on behalf of Salim Al-wasity [salim_alwasity at yahoo.com]
Sent: Wednesday, September 10, 2014 6:00 AM
To: eeglablist at sccn.ucsd.edu
Subject: [Eeglablist] Reference electrode and coherence


Dears
Have a nice day.
According to (Guevara 2005, Fein 1988 and others), Coherency (especially imaginary coherence) might be affected by EEG reference electrode.In my study, an average reference is used and an imaginary coherence (iCoh) is calculated for the components of the averaged EEG signal.Is the averaged reference or the ICA affect the phase of the EEG signals, and hence affect the coherency?

Sincerely
Salim Al-Wasity
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