Christine,

If you simply model the modulation effect at the trial level, the first effect (you call it the "unmodulated effect") is the estimated brain response of that subject associated with the *average* value of the modulation variable. However, that *average* value of each variable (e.g., Confidence) still varies across subjects; so, when you go to the group level, you would still have to handle those "covariates".

To bypass the thorny issue, do this:

1) Obtain the *overall* mean for each condition across all subjects for each of the four variables (Confidence, Response Time, and Accuracy);
2) Subtract the original values by the *overall* mean for each of the four variables from 1);
3) Create the four modulation regressors for each condition yourself by using the values from 2) (the reason for this step is that 3dDeconvolve automatically removes the within-subject mean when generating the modulation regressor);
4) Stick the regressors from 3) in 3dDeconvolve;
5) Perform group analysis with a one-way within-subject ANOVA with the "unmodulated effects".

Gang