Thank you, Donald. I have a related question. As I was running gPPI (this is just for simple contrasts, e.g, A>B; B>A, etc.), I received this message (please note that I added the "..." in the contrast vectors to shorten the number of lines):

Warning:
c is not a proper contrast in +c->Tsp in spm_SpUtil
!!! projecting...
> In spm_SpUtil (line 327)
In spm_FcUtil (line 219)
In spm_contrasts_PPI (line 159)
In PPPI (line 885)
In gPPI_setup (line 133)

from

c=
0
...
0.2500
0
0
...
0
0
0
0.2500
0
0
0
...
0
0
0
0
0
0
-1.0000
0
...
0
0.2500
0
...
0
0
0.2500
0
...
0
0
0

to

c=

0
...
0
0
0
0.2500
0
0
-0.2500
0
...
0
0
0.2500
0
0
-0.2500
0
0
...
0
0.2500
0
0
-0.2500
0
...
0
0
0
-0.0658
-0.0000
-0.0290
0
0
...
0
0
0.2500
0
0
-0.0235
-0.0125
0
0
...
0
0
0.0002 

In the original contrasts when I built my GLM, due to the different numbers of sessions for different conditions, I weighed the vectors so the sum within each vector would be 0. For instance, for an event that appears in 4 sessions, the value would be 0.25 while for another event that appears in 1 session, the value would be 1. It looks like the contrast vectors have been changed when I ran PPI. Does this look problematic to you?

AR