Output of mscohere function

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Satya Gopal 2019-11-12

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https://ww2.mathworks.cn/matlabcentral/answers/490627-output-of-mscohere-function

评论： Satya Gopal 2019-11-13

Hi all,

I have a doubt with regards to mscohere function output in matlab. I have 64 channel EEG data to extract coherence data for. Which means I would need a 64x64 matrix output for a particular frequency band say 8 to 13 Hz. I went through the documentation of mscohere function sveral times. One thing that I saw was that mscohere Cxy part can either be a vector, matrix or 3-D array. When I use the following code to calculate coherence between just two channels, channel 20 and channel 50, I get a 501×1 single column vector:

ch1 = 20;
ch2 = 50;
window = 2*EEG.srate; % create 2 sec epochs
noverlap = 0;    % overlap 
nfft = window*2;    % zero-padding factor 2 for smoothing
fs = EEG.srate;
[Cxy,F] = mscohere(EEG.data(ch1,:),EEG.data(ch2,:),window,noverlap,nfft,fs);
Cxy
Cxy =
  501×1 single column vector
    0.0003
    0.0060
    0.0042
    0.0003
    0.0047 % I copied the first few values only to show the output I am getting

Can someone explain this output to me? Are these coherence values between two channels at 501 different frequency points (I'm just guessing. Not sure!)? Also, how do I input the frequency band of my interest like 8-13 Hz and calculate coherence just for that? Are there more steps in calculating coherence in matlab? How do I get a single value (between 0 to 1) for channel 20 x channel 50 coherence for 8-13 Hz frequency band?

I hope my question makes sense. Please let me know if I need to explain anything further.

Thanks!

Satya

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Answer 1

Daniel M 2019-11-12

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Yes, these are the coherence values between the two channels at the frequencies returned in F. You can manually set the frequencies that you wish, for example F = [8:0.5:13]. The help section indicates this.

%   [Cxy,F] = MSCOHERE(X,Y,WINDOW,NOVERLAP,F,Fs) computes the coherence
%   estimate at the physical frequencies contained in the vector F.  F must
%   be expressed in hertz and have at least two elements.

Then you can average the output to get the total coherence over that band.

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Daniel M 2019-11-13

Because interesting things in EEG don't happen at the just the discrete frequencies, e.g. 8:13. They happen at any frequency. That's why we sample with a smaller step. You can choose a step size that works for you.

Satya Gopal 2019-11-13