1.      In case I'm using differently sized ROIs (e.g. one with 20 voxels, one with 500), does this strongly affect the estimation of the DCM parameters due to the difference in signal-to-noise ratio? 

 

 

I don’t think so – the precision of the observation noise is estimated on a per-region basis (DCM.Ce). However, there may be other effects of ROI size. For instance, if your effect is small (a few voxels) and you have a 500 voxel ROI, then the principle mode of variance in that ROI is unlikely due to be related to your task. That means that the representative timeseries used in DCM for that region may not reflect your effect of interest. So try to pick ROIs where most voxels express the effect of interest.

 

2.      Is there a way to calculate contrasts between connectivity parameters of a PEB? That is, is it possible to compare the connectivity estimates between conditions, e.g. if there is a difference in the strength of modulation of some between-region connection between an emotional or neutral condition?

 

This feature is on my to-do list! At the moment, the most straight-forward way is to form regressors which capture the contrast or difference of interest. Can you give more details as to the regressors in your PEB and your experimental design?

 

All the best

Peter