[FieldTrip] Partial Directed Coherence computation

[Aiello Giovanna]

Good morning,

I am interested in understanding the directed connectivity between the thalamus, where I am recording a Local Field Potential (in 1 channel) and the cortex, where I am recording EEGs (in 16 channels).

My signal is epoched in 4-seconds windows, and I am not interested in understanding the evolution of the directed connectivity in time but rather just which structure is leading which.


I have computed Partial Directed Connectivity (pdc) with the fieldtrip function ft_connectivityanalysis, feeding into the code the transformed, Fourier spectrum (complex double) of my signal (rpttap x channels x frequencies, where rpttap is repetitions (i.e., number of epochs) x tapers).

My final matrix of pdc is 17 x 17 x frequencies (where frequencies are my frequencies of interest). The first 16 channels represent the EEG channels, the 17th channel is the LFP channel in the thalamus. Let’s call this matrix A.
I have two questions:

1) Did I understand correctly that the element A( i, j, f ) for a given frequency f indicates the directed connectivity from element i to element j? (And not the other way around)

2) In my case, A( 1:16 , j  , f ) is a column vector containing the same number!
It looks like, independently on what EEG channel I am considering (unless I am considering the channel in the thalamus), the contribution of directed connectivity to a certain channel j is always the same. This also stays for the thalamus, so if I take A( 1:16, 17, f ), it seems like all the eeg channels have the same directed connectivity to the thalamus. Is that possible? I think it does not really make sense. Please notice that the numbers are not exactly the same, but differ in the 4th decimal digit.



Thank you in advance for your help
Giovanna

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