Dear all,
I have a question regarding the field DCM.X0. I would like to know how this field is created and what it does. My understanding is that it is created during VOI extraction and is used to adjust the extracted timeseries to take into account confounds in the data (e.g. drift). In my study I am running DCM on timeseries from peak voxels that have already been identified in a previous study. As such, I am replacing the timeseries extracted from VOIs identified in SPM with my timeseries of interest. The data used to extract VOIs in SPM are the same that were used to create my timeseries of interest, but different voxels may have been selected.
What should I do about X0 in this case? I have tried using the DCM.X0 that is created by SPM. But perhaps this is problematic, because this DCM.X0 was created based on a timeseries that I am not using in my analysis. I saw in spm_dcm_estimate.m that X0 could be replaced with ones instead. I have tried running my DCM analysis both ways (using X0 created by SPM and replacing it with 1s). If I use X0 created by SPM, the 'actual' timeseries used to fit the DCMs look very strange; there is a huge spike at the beginning and end of my run (e.g. X0default.tiff). If I replace X0 with 1’s, the spikes go away (e.g. X0ones.tiff). Using X0 created by SPM versus replacing X0 also leads to strikingly different results in Bayesian Model Selection.
Please let me know if there is any additional information I can provide. Thanks for your help.
Becky
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