Thanks for the clarifications about stimulus intensity, Lisa. OK, your 3dRegAna script should be fine then. However the interpretation is a little tricky. Assuming you coded both Group and stimulus intensity with 0s (group A, stimulus intensity one) and 1s (group B, stimulus intensity two), AND, assuming you removed the mean from covariate age, then,
intercept (beta0) is the effect of groupA+stimulus intensity one at the average age;
1st beta is the effect of groupB+stimulus intensity one at the average age;
2nd beta is the effect of groupA+stimulus intensity two at the average age;
3rd beta is the common age effect between stimulus intensity one and two;
4th beta is the difference of age effect between stimulus intensity one and two.
HTH,
Gang
