Dear FSL expertsI 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 })
- DCM.Cp (averaged posterior covariance matrix, {\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