Hi Qiuhai,

That's a good observation, that every time point is offset by those same values.
And yes, it is done for the regression.

That is the effect of the -demean option. Every time point has the original mean
subtracted out. It has only the very minor effect on the regression to make the
constant terms from the regression, if one asks for them, more representative
as a baseline value.

Remove -demean from the 1d_tool.py command that makes the motion_deriv.1D
dataset, and you should see the zeros. Also, that will not affect any of your beta
weights of interest (just the constant terms).

- rick