[FieldTrip] Diagonal elements in MVAR connectivity

[Ioana Susnoschi-Luca (PGR)]

Hello!

I am working on effective connectivity analysis with DTF and I am confused about the main diagonal elements - I expected my resulting DTF spectrum to have zero on the main diagonal, but this is not the case. Does it output the power spectrum of the fitted MVAR model instead?

I would appreciate any insight into why this happens/if my thinking is correct. I reproduced my analysis steps (pretty much the same as FT connectivity tutorials) and rationale for diagonal values.

Intuitively, DTF calculates the ratio between inflows from channel j -> i, and all inflows to i, hence I expected i->i to be zero. It uses the transfer matrix output from mvar frequency analysis, but the transf matrix i to i looks like the MVAR model's autospectrum for channel i, so I supposed the diagonal is a ratio between autospectra and sum of inflows to each channel.

Thank you,
Ina

%% CODE

% fit MVAR
cfg            = [];
cfg.order    = 19;
cfg.method  = 'bsmart';
cfg.demean  = 'yes';
cfg.zscore    = 'yes';
mvar_data   = ft_mvaranalysis(cfg, source_ts);

% frequency analysis MVAR
cfg         = [];
cfg.method  = 'mvar';
mfreq       = ft_freqanalysis(cfg, mvar_data);

% DTF spectrum
cfg         = [];
cfg.method  = 'dtf';
dtf         = ft_connectivityanalysis(cfg, mfreq);

dtfPre_dir  = dtf.dtfspctrm;

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