Dear Colleagues,
I have a difficulty in finding the right way to perform with SPM the
following analysis.
1. In a PET study of the brain correlates of emotion, we use 5
experimental conditions, which we consider, on the basis of behavioral
studies,  as stimuli of variable intensity (1, 2, 3, 4, 5)  purported to

induce variable levels  of the emotional state (stimuli of graded
intensity).
2. The second feature of this study is that we want to compare the rCBF
responses  to stimuli of graded intensity of two groups of subjects: (i)

healthy subjects and (ii) patients who experience the emotional state
with decreased intensity (at each given level of  stimulus intensity).
3. The level of emotional response is assessed via a subjective measure,

which may vary from 1 to 9.
4. In each of the two groups,  we have studied the correlation between
the level of stimuli (1, 2, 3, 4, 5) and the rCBF.
5. We have then studied the regions where the correlation between level
of stimulus intensity and rCBF was different in the 2 groups, ie, we
have studied the Group * Level of Stimuli interaction, and found a set
of regions where rCBF grows (or decreases) more steeply in one group
than in the other.
6. The subjective measure of emotion has also been found to be
significantly related to the Group * Level of Stimuli interaction, ie,
the level of emotion grows  more steeply in one group than in the other.

7. However, for each particular region found in (5), we do not know
whether its rCBF is, or is not, related to the differential variation of

the subjective measure of emotion in the 2 groups. Clearly, both the
differential variation of the subjective measure and the differential
variation of rCBF in each region found in (5) are correlated with
stimulus intensity.  But this does not mean that, for all of these brain

regions, the differential variation of rCBF is correlated with the
differential variation of the subjective measure (finding a correlation
between A and C and between B and C does not  mean that A and B are
correlated).
8. Our question, then, is: Which is the way to identify with SPM those
regions found in (5) whose differential variation in the 2 groups in
response to stimuli of graded intensity is correlated with the
differential variation of level of emotional response in the 2 groups?
Thanks for your help!
Serge Stoleru
PS. Is this question equivalent to identifying regions where rCBF is
related with a second-order Group * Level  of stimulus intensity * Level

of emotion interaction? If yes, how is it possible to conduct this
analysis with SPM?