Dear Fred,

This reviewer seems to be referring to the problem of artificial anti-correlations in the context of global signal regression for resting state fMRI data (i.e., resting state data are sometimes orthogonalized to the global mean brain signal using the global signal as a confound regressor in a GLM, prior to the actual functional connectivity analyses). For details, see this paper by Murphy et al.:
http://www.ncbi.nlm.nih.gov/pubmed/18976716

This is not an issue for DCM. DCM for fMRI is only informed about local signals (i.e., those from your regions of interest) and does not consider anything related to global brain signal (unless you manually construct an input that reflects global brain signal).

Best wishes
Klaas

Von: Fred Sanders <[log in to unmask]>
An: [log in to unmask]
Gesendet: Freitag, den 18. Februar 2011, 15:03:32 Uhr
Betreff: [SPM] DCM and global signal

I recently used DCM (in SPM8) to model the connectivity between 2 brain regions. A reviewer was concerned that DCM contains a regressor for global signal and that including this regressor could introduce artificial anti-correlations. I would greatly appreciate it if someone could help me understand this. I did not think DCM used a regressor for global signal - was this done in prior versions of DCM? Or is this referring to the constant term from the first level analysis? Also, how could that introduce anti-correlations?

Any thoughts?

Thanks!!