Michelle,
> is there a way to specify the within x between interaction in the model, or does it do that automatically?
3dMVM automatically models all *possible* interactions between between- and within-subject variables, and there is no way to turn off such interaction in 3dMVM. So if you don't want to model some interaction, 3dLME is more flexible in that regard.
I'm not so sure as to why the model is still failing. Do you mind uploading the input files plus the 3dMVM script and data table so that I could take a close look?
Gang
