Dear Peter,
Thank you very much for your answer. However I would need some precisions to understand the DCM output. I don’t really understand your point concerning scaling. If I compare value in the DCM.Ep.A variable, values are the same as on the graph of the fixed effect which are scale in Hz (see attached image).
- Is the DCM.Ep.A still Hz values?
- Is it only the self-connections which are scaling parameters and other between regions connections are still Hz values?
- Values are scaled relative to what?
- If I understand you example, an self-connection value of -0.3 would mean to have 0.15 Hz (if scaling parameter 1=-0.5 Hz)?
By the way, can you tell me what is the interpretation of the probability for each connection given in the graphical representation of A-probability (on the right of the picture)? Is it to test if the connection is significantly different from zero?
Best regards
Michael Mouthon
-----Message d'origine-----
De : Zeidman, Peter [mailto:[log in to unmask]]
Envoyé : vendredi 5 décembre 2014 11:27
À : MOUTHON Michael; [log in to unmask]
Objet : RE: [SPM] DCM: How to interpret intrinsic connections values computed by DCM12
Dear Michael,
Yes, this is still correct, however there is one important difference in SPM12. To ensure that the self-connections on the A matrix are always negative, these self-connections are now scaling parameters, rather than in units of Hz. That means a value of '1' gives, under the bonnet, a value of -0.5Hz. For anyone interested in the code, this is implemented on lines 105-106 of spm_fx_fmri.m .
Best,
Peter.
-----Original Message-----
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On Behalf Of Mouthon Michaël
Sent: 04 December 2014 18:54
To: [log in to unmask]
Subject: [SPM] DCM: How to interpret intrinsic connections values computed by DCM12
Dear SPM and DCM user,
I would like to have a confirmation of the way how to interpret the intrinsic connections value between regions (DCM.A) compute by the last version of DCM (DCM12).
A previous post from Rainer Bögle in 2011 explained that:
“-connections in DCM are change rates, i.e. a12 tells you how much of the current state/activation (on neuronal level) at time t of region 2 is added to the change in region 1 (at time t). There might be influences from other regions on region 2 which have to be considered, but if if a12 is the sole connection, we can say:
*** a12=0.1 means 10% of the state in region 2 is added to the state (activity) of region 1
and analog for a12=-0.1, this corresponds to a subtraction by 10% of the state in region 2 from the state in region 1.”
https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=spm;7834fcd0.1102
Does this explanation still work with the changes in DCM12?
If it is not the case, how do you interpret the relation between region 1 and 2 if the connection value is positive or negative?
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