> On Aug 2, 2016, at 10:17 AM, Michael F.W. Dreyfuss <[email protected]>
> wrote:
>
> Hi, I have some basic questions about the movement parameters that are put
> out by the HCP preprocessing scripts.
>
> 1) What is the difference between Movement_Regressors_dt.txt and
> Movement_Regressors.txt? I would like to use one as a confound.
The Movement_Regressors.txt are the parameters computed from the motion
correction algorithm. The Movement_Regressors_dt.txt is the same file put
through a detrend command to remove the linear trend and mean.
>
> 2) What does each column in these represent? I'm assuming 6 directions of
> displacement, and 3 for rotation?
The 12 columns represent Displacement {x, y, z}, Rotation {x, y, z}, Backward
Derivative of Displacement {x, y, z}, and Backward Derivative of Rotation {x,
y, z}
>
> 3) What are the units in these and the other movement files in the output?
> mm? voxels?
The units are mm for displacements and degrees for rotations.
>
> 4) I would like to censor time points with too much motion in
> Movement_AbsoluteRMS.txt. Is there a specific movement cutoff you recommend?
> I have made a script that produces a censor file in the fsl format (i.e.
> columns of all 0s with a 1 at the time point that is censored). Is there a
> simple way to add that censor file into the existing analysis scripts along
> with the motion as a confound?
First, I would suggest using the Movement_RelativeRMS.txt file for censoring
instead. The AbsoluteRMS reflects the total movement from the motion
correlation registration target, which is the SBRef volume collected at the
beginning of the scan. In contrast, the RelativeRMS reflects the amount of
motion from the previous time point, which should be considered a more direct
measurement of movement on that specific time point.
I’m not sure to which existing analysis scripts you are referring. We don’t
release analyses of individual scans. So, you should probably include these
censoring confounds as covariates in whatever preprocessing you’re doing for
the individual scans.
After saying all of that, it is worth mentioning that FIX denoising:
- Regresses a set of 24 motion parameters (six rigid body parameters, their
backward derivatives, and the squares of those 12 columns)
- Regresses noise component variance (orthogonal to signal components) from the
scan-level ICA.
In practice, the behavior of FIX is similar to regressing movement paremeters
and voxel-specific censoring (plus other artifact cleanup). If you utilize FIX
preprocessed data, you don’t need to regress motion parameters, and you might
feel comfortable skipping censoring as well.
--Greg
____________________________________________________________________
Greg Burgess, Ph.D.
Staff Scientist, Human Connectome Project
Washington University School of Medicine
Department of Psychiatry
Phone: 314-362-7864
Email: [email protected]
>
> Thank you,
> Michael
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