Dear Andrew, dear SPMers,
> ( This is the gist of it for a simple case. There are additional )
> ( caveats for multiple conditions, unbalanced designs, and )
> ( non-orthogonal contrasts, which we will discuss when necessary. )
referring to your mail from Wed, 19 May, I am wondering how to properly
perform a mixed effects analysis in a study with multiple conditions.
Due to the large amount of data (11 subjects, about 1500 timesteps per
subject) it seems impossible to do a group analysis using a fixed
effects model (because of memory problems). Therefore, I firstly fitted
the model for each subject, and now I want to feed appropriate
contrast/condition images into a second level mixed effects analysis. I
have 7 conditions (epochs):
B(aseline) A(ctivation) C(ontrol) A1 C1 A2 C2
Firstly, I am interested on the comparisons A versus C, A1 versus C1 and
A2 versus C2.
Secondly, I want to determine specific (and perhaps also general)
effects of the interventions 1 and 2 on ‘A versus C’, for example using
the following interaction contrasts:
0 -1 1 1 -1 0 0
0 -1 1 1 -1 0 0
0 0 0 1 -1 -1 1
0 0 0 -1 1 1 -1.
Finally, I would like to show that the task component 1 (tested for by
the contrast C1 versus C) and task A per se (A verus C) produce similar
activations (anatomically, not necessarily quantitatively). (The same
applies to possible general effects of the interventions 1 and 2.)
My first questions are: for the latter purpose, do I have to use a
conjunction analysis or, perhaps, masking? And if a conjunction would be
more appropriate (by the way, why?), how do I have to set the thresholds
to obtain a significance level of p < 0.05, corrected (both in single
subjects and in the sort of group analysis proposed by Karl in a recent
mail from Thu, 2 Sep)?
Secondly, what about the additional caveats for multiple conditions and
non-orthogonal contrasts mentioned in your mail from 19 May? And,
generally, should I enter contrast images or specific condition images
into the second level of analysis?
Any help on this would be appreciated. Thank you very much in advance.
Best regards - Oliver
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