Dear Yang,
the uU in spm_peb_ppi.m defines your [nx3] PPI matrix, where n is the number of your regressors.
let's say you have 4 regressors [A B C D] , no parametric modulation, and you want your psychological variable to be C-D. in this case your [nx3] matrix would look like this: [ [1:4]' [1 1 1 1]' [0 0 1 -1]'].
hope this helps,
Sven
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