# 3dDeconvolve - 'combined' contrast is opposite sign of individual ones?

Hi AFNI experts,

I just had a puzzling result for a few subjects when using 3dDeconvolve and wanted to check my understanding.

I have a breath hold task for which I have 3 conditions - breathing normally, cued breathing, and breath hold. In my 3dDeconvolve command I specified 3 contrasts, one for comparing the hold to each breathing condition, and one to compare it to both breathing conditions. The afni output correctly specified the matrices like this:
1 -1 -1
1 -1 0
1 0 -1

these were specified like:
-gltsym ‘SYM: hold -instructed -normal’ -glt_label 1 hold_gt_breathing
-gltsym ‘SYM: hold -instructed’ -glt_label 2 hold_gt_instructed
-gltsym ‘SYM: hold -normal’ -glt_label 3 hold_gt_normal

Somehow it appears that in many voxels, the sign is flipped for the third contrast as opposed to the first two (ie the coefficients for 1 and 2 will be positive, and 3 is negative). Does this make sense and could someone explain under what conditions this would happen if so?

Thank you!
James

Suppose that the effects of the three conditions are A, B, and C. If B < 0, you would have A > B + C and A > B while A < C. For example, (A, B, C) = (2, -1.5, 3) or (-1, -2, 0).