Hi Gang,

Thanks for your reply. The reason to residualize all data (even the RTs) is to control potential confounds (such as session order). For instance, the two different treatment conditions (DD and PC) were given in either session 1 or 2 (was counterbalanced across subjects). Because the same task was given in two sessions, participants' RTs were generally lower in session 2 than 1 (irrespective of the treatment). So if RT somewhat differs across the sessions, I guess that would affect the distribution of RTs in each treatment condition (e.g., DD condition RT: generally high RTs from subjects had DD in session 1 but low RTs from subjects had DD in session 2). This is partly why I wanted to residualize RT as I would do for the brain data.

I also have tried with raw RT with and without covariates in the 3dLME, they look somewhat similar for some correlations that I checked.
But given the concern I have above, I thought I would have to control the factor somehow.

Best, Michelle