Dear Danila,
No. With [1 -1 1 0 0 0 0 0 0] you test whether (A - B + C) > 0 in case of a T contrast or whether (A - B + C) is sig. different from zero in case of an undirected F contrast. You can also think of "Is the sum of A and B larger than B". There are instances in which such a comparison makes sense, but usually it's not what one wants to look at
It seems you're interested in
1) voxels/clusters associated with higher activations for A relative to B, this would be [1 -1 0 0 0 0 0 0 0]
2) voxels/clusters showing a positive linear relationship with regressor C, which would be [0 0 1 0 0 0 0 0 0]
3) the conjunction of 1) and 2) = voxels/clusters with an effect A > B AND a positive correlation with C. For setting up a conjunction, you would first specifiy the contrasts for 1) and 2). Then press the Results button once more, select the two contrasts (while holding [control]) and choose "conjunction". However, it has been argued on the mailing list that these conjunctions are statistically invalid for within-subject designs. As a solution, you can save the SPMs for the two contrasts at a certain threshold and generate an image that shows the overlap. This is no proper statistical test of course.
Best,
Helmut
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