To add a little more to Michael's comment...
Motion regressors are an additional correction to the realignment calculation. They can correct for
a resampling error caused when images are resliced to align the voxels. Friston, et.al. (1996)
called this a motion adjustment correction, and Grootoonk, et. al (2000) directly showed it was a
resampling error. Often 6 linear motion regressors are used, although SPM5 mentions 12, and the
Friston paper had 24.
Resampling is a mathematical property, while unwarping should correct a physical deformation
effect. If uncorrected physical deformation causes some linear effect with position, I suppose the
motion regressors may possibly pick it up. Motion regressors can erase activations if there is task-
locked motion (nice example in Lund, et. al (2005)) as Michael stated before.
- Paul
On Tue, 30 Jan 2007 18:42:37 -0000, Laura Mancini <[log in to unmask]> wrote:
>My understanding was that it depends if you use just the "realign" button or the "realign and
unwarp" button.
>I wouldn't be able to explain exactly why, but in my understanding if you choose "realign", than
SPM translates and rotates the images but doesn't do anything else, so that you have to put the
regressors in the first level model to model out the actual effect of the movement.
>But if you use "ralign and unwarp" then SPM not only rotates and translates the slices to realign
them, but also correct for the changes expected in the signal due to these movements, so you
don't need to add the rp-file in the first level model.
>
>I hope I got it right, maybe someone can confirm this.
>
>Cheers,
> Laura
>
>
>-----Original Message-----
>From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On Behalf Of Glabus,
Michael
>Sent: 30 January 2007 14:27
>To: [log in to unmask]
>Subject: Re: [SPM] motion correction
>
>
>I'll have a stab at this.
>Neuroimaging involves digital sampling and fMRI images have rather course spatial sampling
resolution. Resampling during realignment involves some form of interpolation and that will
introduce errors - think about partial volume effects.
>
>The realignment procedure uses an iterative procedure to minimize the error between a template
image and the target image. By definition, the minimizing of the error means that some residual
misalignment error may exist. Modeling the motion parameters as regressors, while reducing a dF
for each, *should* help in reducing the error term in the linear regression model. You could try
with and without to see if there's any noticeable improvement.
>
>Cheers - Mike
>
>>-----Original Message-----
>>From: SPM (Statistical Parametric Mapping)
>>[mailto:[log in to unmask]] On Behalf Of Markus Burgmer
>>Sent: Tuesday, January 30, 2007 6:41 AM
>>To: [log in to unmask]
>>Subject: [SPM] motion correction
>>
>>dear spmers.
>>
>>i searched the archives about a question about motion correction and
>>covariate for the model, but did not find a sufficient answer
>>(some were to
>>technical for me to understand, sorry about that).
>>i always understood realignement in spm in the way, that
>>afterwards (after
>>reslicing the images) spm corrected for the motion between the
>>images. for
>>me it sounds like after that, almost no differences in motion
>>between the
>>images exists, the images almost "look" like one static image over the
>>whole time series. (maybe here i am wrong, please correct me).
>>now i read in many mails, that it is recommended to integrate the
>>translation and rotation data of the rp-file as a covariate
>>into the model
>>to control for movement effects.
>>
>>now my question:
>>if both of my above assumptions are correct, why should i
>>integrate the
>>motion covariate? didn't i control for movement artefacts by the
>>realignement procedure itself? i thought that the rp-file gives me
>>information about the rotation and translation which results
>>in the new
>>images after realignement. integrating those also in the model
>>would be one
>>step to much?
>>
>>if i still have to integrate the regressor, did i understand
>>it right to do
>>it in the first-level procedure?
>>
>>thanks in advance
>>
>>markus burgmer
>>university münster, germany
>>
>
>
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