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Dear FSL experts
I estimated spectral DCM, fully connected with 8 regions, and another
spectral DCM with some connections off. I did it separately for all my
subjects. After DCM estimations I used the following code. For example I
choose only 2 subjects, so N=2

GCM = {'DCM_Subj1_full_modelFull_estimated.mat',
'DCM_Subj1_fullTimeSeries_modelAlternate_estimated.mat';

       'DCM_Subj2_full_modelFull_estimated.mat',
'DCM_Subj2_fullTimeSeries_modelAlternate_estimated.mat'};

N=2;

% Specify PEB model settings (see batch editor for help on each setting)

M = struct();

M.alpha = 1;

M.beta  = 16;

M.hE    = 0;

M.hC    = 1/16;

M.Q     = 'single';



% Specify design matrix for N subjects. It should start with a constant
column

M.X = ones(N,1);

 % Choose field

field = {'A'};



% Estimate model

PEB = spm_dcm_peb(GCM,M,field);

save('PEB.mat','PEB');


At this stage, I tried to search over nested PEBs using (
https://en.wikibooks.org/wiki/User:Peterz/sandbox)

BMA = spm_dcm_peb_bmc(PEB);


However I got an error
Error using spm_dcm_peb_bmc (line 84)
Please provide a single PEB model


I tried using comparing the full PEB to nested PEB but still got this error
BMA = spm_dcm_peb_bmc(PEB, GCM(:,1));
Error using spm_dcm_peb_bmc (line 84)
Please provide a single PEB model

Finally I used the following command and it worked
BMA = spm_dcm_peb_bmc(PEB(:,1));


Now I have 2 questions

1. Why I can only run PEB over models separately ? that means where I use
PEB(:,1), I have my subjects from FullModel, and when I use PEB(:,2) I have
my subjects from alternate model. How to search over nested PEBs in this
case ?


2. Even with running on two subjects, I got some results, how do I know
where these results are significant ?
My BMA is as follows
BMA =

  struct with fields:

    Snames: {2×1 cell}
    Pnames: {64×1 cell}
      Pind: [64×1 double]
    Xnames: {'Covariate 1'}
         M: [1×1 struct]
        Ep: [64×1 double]
        Eh: 0.7110
        Ch: 0.0354
        Cp: [64×64 double]
        Ce: [64×64 double]
         F: -1.5468e+05
        Pp: [64×1 double]
My BMA.Ep is almost empty other than 3 variables, which are of the
order-0.03 etc. what does that mean ? How high should the value be to
consider that it exists
I know that following are the definition of the variables

   - DCM.Ep (averaged posterior means, {\displaystyle \mu }[image: \mu])
   - DCM.Cp (averaged posterior covariance matrix, {\displaystyle \Lambda
   ^{-1}}[image: {\displaystyle \Lambda ^{-1}}])
   - DCM.Vp (averaged posterior variance vector of each parameter - the
   diagonal of DCM.Cp)
   - DCM.Pp (averaged posterior probability of each parameter).


However I m still confused whether to identify the strength between two
nodes, I would look at BMA.Ep or BMA.Cp ?

3. In the BMR window, there is a graph called posterior, It has 8 bars, and
bars values are between 0.7 and 0.3, what does that mean ? It is perhaps
connection strengths but can you please confirm and what strength would be
considered for existence and what would be considered negligible ?



Any help will be very useful
Thank you