afni.nimh.nih.gov

If you only have one group of subjects, the population-level analysis would be just comparing the aggression effect between the two conditions of mean and nice? If so, that would be a paired t-test using 3dttest++. If you directly obtain the contrast of the aggression effect from each subject, that would be a one-sample t-test using 3dttest++ or 3dMEMA.

Gang