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Dear SPMers, I am beginning to use SPM and DCM and I have a few remedial questions about DCM. We have analyzed some PPI data that suggests a relationship between regions and we would like to use DCM to establish a model for connectivity. I've used the example photic/motion/attention model that you've published on your website, I've looked at the old ppts that you've posted from your yearly courses on SPM, and I think I have a much better understanding of the theory behind DCM. But I'm having some problems applying it: I'm having trouble designing a DCM model. We have a fingertap experiment in which we ask subjects to press a button in response to a stimulus. The stimulus conditions vary - we have a regularly timed stimulus (beat), a randomly timed stimulus (random), and a rest period in which there is no stimulus (rest). 1)Does this seem like an appropriate way to specify the regressors for a DCM analysis? I then find the VOIs. I specify a DCM using my VOIs and ask SPM to estimate a model using the connections I specify. MATLAB returns several errors after calculating the first PEB and GNS iterations: Warning: Returning NaN for out of range arguments In spm_Ncdf at 76 In spm_dcm_estimate at 72 In spm_dcm_ui at 312 This first GNS iteration value is functionally zero (e-22). As a consequence, when I review the DCM my modelled system output is flat. 2)What does this mean, why do I get such low values and why does the program return an NaN? Is it a function of the data? Does it mean that I've designed the model inappropriately? Or I'm not properly defining the DCM? Any help is greatly appreciated. Thank you in advance.