Dear Alfonso and fellow forum users,

I have a question regarding the use of contrasts in a second-level setting. Here are the between-subjects factors and conditions in my study:

Between-subjects: "GroupA," "GroupB," "Age," "BehavioralScore_GroupA," "BehavioralScore_GroupB"
Conditions: "rest"

Based on previous discussions, the contrast for the interaction effect is specified as [0 0 0 1 -1], with control for age in the regression model. The regression equation looks like this:

Functional Connectivity = Beta0 + Beta1(Group) + Beta2(Age) + Beta3(BehavioralScore*Group)

Now, I'm wondering if I can include the main effect of "BehavioralScore" in the regression, as shown below:

Functional Connectivity = Beta0 + Beta1(Group) + Beta2(BehavioralScore) + Beta3(Age) + Beta4(BehavioralScore*Group)

In this case, the factors are defined as follows:

Between-subjects: "GroupA," "GroupB," "BehavioralScore," "Age," "BehavioralScore_GroupA," "BehavioralScore_GroupB"
Conditions: "rest"

The contrast for the interaction effect is [0 0 0 0 1 -1].

However, when I run this model, the GUI displays a warning message and suggests simplifying the second-level model. I'm wondering why is this warning happening?

Thank you for your assistance and valuable suggestions.

Sincerely,
Fei