> Do you know why it produces a result even without the interaction effect being in the model? Would you expect the result to be different?

3dLMEr would not parse the specification properly even though it didn't throw an error in this case. To make sure you get the intended results, rerun the script with corrected coding.

> 1. Is there an association between imaging and Outcome1?
> 2. If yes, does this association get stronger or weaker over time?
> 3. If it changes, is the magnitude of change different between the groups?

In your model specification "Time*Group+Age+Gender+outcome1+outcome2+outcome3+(Time|Subj)", no interaction is assumed between Time and outcome1. Thus, to answer the three questions above, you don't need to get Time involved. For example, for the question "does this association get stronger or weaker over time?", the answer is that the association does not change because you already assumed so in your model.

If you really want to address these three questions, account for the interaction between Time and outcome1 by creating a regressor of their product after proper centering.

Gang