Dear Klaas and SPMers,
I am trying to setup a convenient DCM model for a 2x2 factorial design,
and I would very much appreciate your advice to be sure that what I am
doing is correct.
The model is constituted by 4 anatomical regions, with only one region
receiving a driving input.
There are two different questions that I would like to address:
1. Is the overall functional integration within the system stronger for
stimulus A vs B (main effect of stimulus (A1+A2)-(B1+B2))?
2. Is there a significant interaction between the two factors, such that
the connection strengths for stimulus A vs B are stronger in the context
of task 1 vs 2 (interaction (A1-B1)-(A2-B2))?
To address these two questions I have set up a GLM Design matrix with
two conditions:
- onset of events of condition A (representing the main effect
(A1+A2)-(B1+B2)) -> 'STIMULUS'
- onsent of events of condition 1 (representing the main effect
(A1+B1)-(A2+B2)) -> 'TASK'
I then defined a DCM model, in which 'STIMULUS' provides the driving
input, and 'TASK' is allowed to modulate all the defined intrinsic
connections.
Is the procedure described above so far correct?
Do I understand it correctly that:
- The intrinsic connection strengths (matrix DCM.A) represent how the
main effect of 'STIMULUS' that is induced in the input area is conveyed,
via the specified connections, to other areas (answer to question 1 above).
- The modulatory connection strengths (matrix DCM.B) represent the
degree to which the activity is passed from the input region to other
areas, depending on the level of 'TASK' (answer to question 2 above).
Thank you for your help!
Best wishes,
Marco
--
Marco Tettamanti, Ph.D.
San Raffaele Scientific Institute
Facoltà di Psicologia
Via Olgettina 58
I-20132 Milano, Italy
Tel. ++39-02-26434888
Fax ++39-02-26434892
Email: [log in to unmask]
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