Hi Peter,
Thanks for the reply. The dataset I am applying to is fMRI and not resting state.
Does that mean that dimensionality reduction affects the estimation and Ep.B, Ep.C values in negative way? Because Across 9 VOI's, I am planning to setup around 60 models, and then perform BPA to study which connections have highest strength. Does setting large model space affects the parameter estimates in negative way?
Thanks a lot
-----Original Message-----
From: Zeidman, Peter [mailto:[log in to unmask]]
Sent: Thursday, July 17, 2014 5:35 AM
To: Kapse, Kushal Janardan; [log in to unmask]
Subject: RE: [SPM] DCM, max#VOI's
Hi Kushal,
The number of ROIs you can have is just a matter of what's practical - there's no hard limit (if you're using a script).
If you have more than nmax regions, then a dimensionality reduction is performed prior to estimation (by modifying the priors, it seems).
If you prefer, you could try setting nmax to 9 so this doesn't occur (DCM.options.nmax=9), however you will need to be conservative with which connections you allow , or you could end up with more parameters than are sensible. You could also try using the new "Estimate (cross-spectra)" option in the GUI, if your data is resting state, which is specifically designed for large models (see "A DCM for resting state fMRI", Friston et al. Neuroimage 2014).
Best,
Peter.
-----Original Message-----
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On Behalf Of Kushal Kapse
Sent: 16 July 2014 17:15
To: [log in to unmask]
Subject: [SPM] DCM, max#VOI's
Hi SPM users,
I have a confusion/question regarding DCM max# VOI's to be included in the DCM model specification. I do know that in GUI, DCM allows 8 VOI's only. But there is a provision where i can change this nmax to greater numer. I am using 9 VOI's to specify my network and i am using the script 'dcm_spm8_batch.m' and editing it to specify my models. I have following two questions about DCM nmax VOI's
1- As i am using script to run DCM analysis, do i need to worry about nmax as my VOI's are 9 and the GUI limit is 8 (which i assume also should apply to script)???
2- If there is a need to change the DCM code in a way that i can increase nmax to 9 or above, which specific script do I modify?????. The current script doesnt show any option for that
DCM.options.nonlinear = 0;
DCM.options.two_state = 0;
DCM.options.stochastic = 0;
DCM.options.nograph = 1;
I do know someone earlier did post this within current or last year and i did try to search on the thread, but i wasnt able to get this info.
If anyone can throw the light on this, that would be helpful.
Thanks
Kushal
|