I have two questions regarding DCM.
My experiment: I have a 2x2 factorial design with the factors movement (active/passive) and feedback (visual/audiovisual). Subjects move their hand (active condition) or their hand is moved by a machine (passive conditon). Feedback about the movement is given either by visual feedback (visual condition) or by both auditive and visual feedback (audiovisual condition). Data was acquired in 5 sessions separated by a short break. Data acquisition was stopped between sessions. I therefore have to concatenate the sessions before extracting the eigenvariates.
My questions are:
1. I concatenated the sessions before extracting eigenvariates using two different approaches. First, I used spm_fmri_concatenate. Second, I modelled the session regressors in first level (similar to the PPI exercise in SPM manual). The results are similar, but they are quite different from the original results (without concatenation). Is this possible? If so, what would be the way to proceed?
2. In my first DCM analysis I am interested in connectivity differences during active and passive movements leading to visual feedback. I do not want to analyze the influence of audiovisual feedback. How to proceed best? In my models, I can not simply neglect the audiovisual condition because they might be influencing activity in the visual cortex. A first option would be to model the audiovisual feedback in the first level design matrix and use, during the extraction of the eigenvariates, an effect-of-interest contrast that excludes the time series variability related to the audiovisual conditions. In my DCM analysis, I then do not have to account for the effect of audiovisual feedback anymore. A second option would be to extract the time series without trying to remove the audiovisual effects, but instead model these effects for instance as direct inputs (C-matrix) in the visual and the auditory cortex (and perhaps also in the B-matrix), but considerung this parameter as parameter-of-no-interest. What would be your suggestions on this?
this is a classic question for the spm list. check their archives first and then ask them if you don't see an answer (although i'd be surprised if this hasn't been asked before)
Thank you very much for your suggestion. I already went through the spm mail list, but those I've read didn't provide me with much information. I also sent the questions to the mail list, but didn't get an answer. Therefore, I thought I might try here.
Dear Belkis, can you share your final solution for DCM with multiple sessions? Thanks
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