Von: "Curcic, B (med)" <[log in to unmask]>
An: Klaas Enno Stephan <[log in to unmask]>
Gesendet: Montag, den 11. Oktober 2010, 15:45:13 Uhr
Betreff: RE: [SPM] DMC Question - Please help!!!
Dear
Klaas,
Thank you for your answer.
I understand that both (0.2 and 60%) are equivalent in a mathematical way (and I understand the unity restriction for inputs). However, phenomenologically, the absolute values of coupling parameters (such as 0.5 or 1.38 etc) standing alone are not adding to our information because those values are restricted to the particular DCM model. As far as I understand only relative values (to those within the same model) are of importance. For example, if I look at the same areas, and coupling parameters for the same connections between models, they are different (say 0.72 in DCM1 and 0.52 in DCM2). Those coupling parameters vary more between models than between subjects within the same model.
Therefore, I am not sure that summing them add to overall information. However looking at the relative value such as 60% has a phenomenological meaning.
Or do I miss something in my reasoning?
Thank you very
much
Kind regards
Branislava
--
Branislava Curcic-Blake, PhD
Cognitive Neuropsychiatry group
BCN Neuroimaging Center (NIC)
University Medical Center Groningen
Antonius Deusinglaan 2
9713 AW Groningen
Tel: +31503638806
Fax: +31503638875
[log in to unmask]-----Original Message-----
From: Klaas Enno Stephan [mailto:
[log in to unmask]]
Sent: Mon 11-10-2010 15:14
To: Curcic, B (med);
[log in to unmask]Subject: RE: [SPM] DMC Question - Please help!!!
Dear Branislava,
These interpretations are equivalent. You could either say that the
presence of
the modulatory input (assuming it has an amplitude of unity) reduces the
effective connectivity from 0.5 to 0.2 Hz, or that it leads to a reduction of
effective connectivity by 60%.
Note the dependence on the amplitude of the modulatory input. This becomes
particularly important when dealing with parametric regressors whose amplitudes
change over time (e.g., models of trial-by-trial learning).
Best wishes
Klaas
________________________________
Von: "<Branislava> <Curcic-Blake>" <
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An:
[log in to unmask]Gesendet: Montag, den 11. Oktober 2010, 11:00:03 Uhr
Betreff: Re: [SPM] DMC Question - Please help!!!
Hello Fred and Fu,
I am not sure that A's and B's should be added like that. In
principle they have
the same dimensions, (1/second) but the
meaning of summing or subtracting them
is not clear to me specially when their absolute values are as far as I
understand not important. As far as I understand their relative value is
meaningful, so in your case Fred, you would be interested in their ratio
(-0.3)/(0.5)= -0.6. That can be interpreted as 60 % decrease of the effective
connectivity due to the modulatory effect.
Kind regards
Branislava
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