i received an offline answer about my question concerning parametric
modulation which i like to allocate.
the recommendation was, to perform a conjunction analysis with 2 t-
contrasts (a<b, b<c) for each subject at first level and to enter the 2 con-
images per subject into a 2x2 ANOVA (2 groups, 2 images) at second level.
with this i do not have to assume a true linear increase stimulation load
between the 3 conditions (a,b,c).
thanks for the help