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Dear Dr. Peter Zeidman,

Thank you for your email. My confusion was whether all the best models could have the very same structure. Thank you for your help clearing my confusion. I just have a couple of questions more.

In my results, I find with respect to intrinsic connectivity, the two regions that were not connected, have a Bayesian parameter average under selected model value as -32, which as I understand from this post (https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=ind1412&L=spm&P=R18218&1=spm&9=A&I=-3&J=on&d=No+Match%3BMatch%3BMatches&z=4) is log scale parameters (Model posterior value = 0). I would like to confirm if a high negative value thus would mean an (almost) 0 probability.

Using the same formula for conversion to rate changes, if I understand correctly, I would find that all the intrinsic parameters have negative values (or zero). Also, I get negative values in the modulation matrix and if I understand correctly from your conversation that negative modulation and negative intrinsic connections would add up (https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=ind1407&L=spm&P=R40113&1=spm&9=A&I=-3&J=on&d=No+Match%3BMatch%3BMatches&z=4). The thing I am not sure about, is whether I should consider the negative rate constants or the log scale parameters for such an interpretation.


Also, it seems that the graph overlay of intrinsic connections produced by  spm_dcm_graph uses the log scale parameters and thus suggests a strong connection between two regions that have a value of -32. I am not sure what this might suggest. I would really appreciate help interpreting this result.

Thanks again,

Best regards,
Atesh





On Mon, Mar 9, 2015 at 3:51 PM, Zeidman, Peter <[log in to unmask]> wrote:

Dear Atesh,

I hope you don’t mind me CC’ing the SPM mailing list so that other can benefit. You emailed regarding your family definition for post-hoc DCM. You were surprised that whichever family you had first in your if…elseif statement in your family definition function, your models would get assigned to the first family.

 

The role of the family function is to map each individual candidate model to a single family. The free energies of the models in each family are then pooled to allow families of models to be compared. I think your confusion is a simple matter regarding the use of “if” and “elseif” statements. When the first statement in your family function is:

 

if B_matrix(regionC,regionA,input_index) ~= 0

    family = 1;

Your best models are being assigned to family 1, and no other family, because your best models have the A->C connection. If you change this to:

 

if B_matrix(regionD,regionA,input_index) ~= 0

    family = 1;

 

I expect that some or all of the same best models as before are getting assigned to the new family 1, because these models have both the A->C connection and the A->D connection.

 

Let me give you another example of where this will happen. See lines 76-79 in your family function:

 

elseif B_matrix(regionB,regionE,input_index) ~= 0

    family = 8;

elseif (B_matrix(regionB,regionE,input_index) && B_matrix(regionB,regionE,input_index)) ~= 0

    family = 9;

 

The second elseif statement can never be executed, because if B_matrix(regionB,regionE,input_index) ~= 0  is true, then the assigned family will be 8, and Matlab will never reach the next line of code. This is the definition of “elseif”.

 

So, you need to think about how you can define families such that every model is only assigned to one family.

 

Please direct any further queries to the SPM mailing list, where one of my colleagues or I will reply.

 

Hope that helps,

Peter

 

From: atesh koul [mailto:[log in to unmask]]
Sent: 09 March 2015 13:26
To: Zeidman, Peter
Subject: Re: DCM post-hoc family inferences

 

Dear Dr. Peter Zeidman,

Thank you for your email. The concern that you have mentioned is correct.

The problem is actually the same. However, since the data is from a lesser number of participants, the winning modulatory connection is different. So, in the same data that I have sent you, if a modulatory connection from region A to B is tested first (as in the attached which_family2.m file), the user defined family is selected to be family 1. Alternatively, if a modulatory connection from region  A to C is checked first, the result is again family 1 (basically all the connections that have DCM.Pp.B value >0) .

Thanks again,

Best regards,

Atesh