covariable

Dear SONG Xiao-Wei and YAN Chao-Gan:

Thank you for making REST fmri analysis software available. It has
been very helpful in my analyses.
I have a question about how the software analyzes covariables.

I notice that you add both a linear and a mean regressor to the
covariable matrix.
I understand why you add the mean, but why is the linear regressor also added?
Wouldn't this have been taken care of by detrending the data?

thank you,
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Darren Gitelman