| I ran SPM99 "Multi-group:conditions and covariates" comparing the
| between 2 conditions in 2 different groups as described in
| The study is attempting to identify hemisheric asymmetries in group 1 and
| 2. For each subject in both groups their normal scan is condition 1, and
| flipped about the x-axis is used for condition 2. Therefore, I have come
| with the following:
| group 1: differences between flipped vs nonflipped
| group2: differences between flipped and nonflipped
| The results of contrast [1 -1] and [-1 1] appear to be the mirror image of
| other about the x-axis. Is this correct? Am I using the correct
| look at the differences between group 1 and 2 for flipped vs not flipped?
| does one interpret the contrasts?
Assume that the difference images are flipped-unflipped, then the left side
image shows left-right, and the right side shows right-left. What appears
right of the difference image is the negative of what appears on the left.
the difference on the left be called (l-r)_1 and (l-r)_2 for the two groups,
corresponding pixel on the right side of the images are -(l-r)_1 and
For the l.h.s, the first contrast is therefore: (l-r)_1 - (l-r)_2
The second l.h.s contrast gives: - (l-r)_1 + (l-r)_2
The first contrast on the r.h.s shows: (-(l-r)_1) - (-(l-r)_2) =
-(l-r)_1 + (l-r)_2
and the second on the r.h.s shows: -(-(l-r)_1) + (-(l-r)_2) =
(l-r)_1 - (l-r)_2
As you can see, the first contrast on the l.h.s is the same as the second
on the r.h.s. Similarly the first contrast on the r.h.s is the same as the
on the l.h.s.
In effect, this analysis is a random effects model, since all the variance
estimated between subject.
I hope this reply has clarified rather than confused.
All the best,