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Dear Gaetan, In principle, the implicit assumption behind SPM source reconstruction of multiple conditions is that the same set of sources is activated in all conditions, just to a different extent. Thus, it might be problematic to compare auditory with visual stimulation, for instance. Also if you invert two conditions separately there will be separate mean scaling applied to each so the interpretation of your statistical comparison will be somewhat different from inverting together. Finally, without constraints to a common set of sources you are likely to lose sensitivity because your sources will only weakly overlap. So unless it is indeed the case that you expect completely different activations in the two conditions (in which case I don't see the point of comparing them), you should invert them together. Best, Vladimir On Wed, Mar 2, 2011 at 9:01 AM, Pierre Larigneux <[log in to unmask]> wrote: > Dear Vladimir, > > first, I wanted to thank you about the advice you gave me for the analysis > of my data. > Before finnish to write my article, I got a last question on which we have > an intense debate with my supervisor. > > I have two conditions that I inverted together in order to extract the > common pathway between them and in order to compare them with a statistical > test. (as recommanded in the manual) > In order to identify, specific pathway for each of my condition I inverted > separatly the conditions. I found slightly different pathway which are > indeed confounded when I invert them together and I think it is really > interesting. > > My question is : in what extent can I compare the source obtained for both > condition separatly given that the group based algorithm did not give me > exactly the same pattern of sources? > Can I apply a t-test between them? > Is there a sense to do that? > > once again, thank you for your precious help. > > Gaetan Yvert >