[FieldTrip] Phase synchronization and coherence spectrum

[Laurent Sheybani]

Dear Fieldtrippers,

I am trying to implement the example provided here (http://www.fieldtriptoolbox.org/tutorial/fourier/) to measure the phase synchronization between two signals. I followed the paragraph starting by "The coherence spectrum".

>From what I understand of the "Coherence spectrum" example script of this web-page, the plot that one should obtain when calling plot(coh) is a 2-D plot with the y-axis corresponding to the coherence index between 0 to 1 and x-axis corresponding to frequencies (right ?).

However, when I changed the length of the signal (argument "len"), I noticed that the frequency (x-) axis increased by the same factor (e.g. if I change len from 1 to 2.5, the x-axis changes from 100 to 250). The coherence is no more on 10 Hz, but on 25 Hz. I must have missed something simple, but I do not understand why the results (& frequency axis) depend on the duration of the signal ? Did I miss a normalization somewhere ?

Any help would be greatly appreciated !
Thanks in advance !

-----------------------------
Dr. Laurent Sheybani, Ph.D
Médecin interne en Neurologie Clinique
Clinique de Neurologie
Hôpitaux Universitaires de Genève
Rue Gabrielle-Perret-Gentil, 4
1205 Genève
Switzerland

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