Dear SPMers,

	Thanks for your response, Jesper!  I have some additional 
questions that I thought I'd post to the group.  

	I've been seeing motion in the 2-4 mm range and am worried that it might 
cause artifacts in the data.  For example, there is a "ring" of 
deactivation in the white matter when I compare one of my event-related 
trial types to baseline.  Could that be caused by motion?  If so, then 
why wouldn't there be a ring around the edge of the functional image, 
too? (I'm not seeing a ring around the edge of the brain, either for 
activations or deactivations).

	I'm also wondering how much of this motion (i.e., 2-4 mm) is corrected 
by the realignment routine and how much "extra" motion would be 
correected by including the realignment parameters as a covariate during 
model estimation.

	I've also heard that some people will throw out subjects whose motion 
is greater than 2-3 mm.  Is that a general practice of most people who 
use SPM?  

	I've also heard that motion greater than 10% of FWHM is 
problemmatic because it introduces false activations.  So, for example, 
if I smooth my data to 10 mm, then motion greater than 1 mm would start 
to produce false activations and false deactivations.  I'm wondering how 
people generally handle this.  Do people throw out subjects whose motion 
is greater than 10% of FWHM?  

	Also, do most SPMers use a bite bar?  In my experience, motion is 
much reduced by using a bite bar.  Do others find that too?  How do bite 
bars and vacuum packs compare?

	Also, I forgot to say earlier that I'm using an 
event-related design (1 stimulus every 16 seconds).  Are event-related 
designs less susceptible to motion?  I've heard that they can be, but I'm 
not sure why this should be the case.


	Finally, I also forgot to say that the motion in my experiment is the 
total motion across eight 6-minute runs.  The data from all 8 runs is 
being analyzed together during model estimation.  In any case, if a subject 
moves 4 mm, then it's usually the case that they moved 0.5 mm per 6 minute 
run rather than 4 mm in a single run.  Does that make any difference?  Is 
it better to have motion spread out across multiple runs than 
concentrated within a single run?  I would think not, in this case, since 
I'm averaging the functional data from all 8 runs together, but I figured 
I'd ask. 


Thanks,

:> Daniel


> > Dear SPMers,
> >
> >         I was wondering whether anyone has any ideas about how much motion
> > is too much motion.  In a current study that we are conducting, we are
> > collecting functional data with a voxel size of 3.75 X 3.75 X 5 mm.  When
> > realigning a subject's functional data from this study, is 2 mm in the x-,
> > y-, or z- directions too much motion?  What about 3 mm,or 10 mm?  SPM
> > produces a postscript file that indicates how much motion it has estimated
> > during realignment, but would realignment work well if there was too much
> > motion?  Is there an amount of motion (e.g., 2 mm, 3 mm , 5mm) at which
> > point realignment becomes less successful?  Or, is it always equally
> > successful?
> >
> 
> I think you can say that realignment is always (almost) succesful in terms of
> finding pretty correct estimates of the movement parameters.However, it is
> clear that the realigned data are not identical to those you would have
> collected had there been no motion (See Friston et al, MRM 96;35:346-355).
> There are a number of tentative explanations for why we cannot recreate data
> correctly including i) interpolation errors, ii) intra-scan movements and
> iii) movement-by-suceptibility interactions. Of these error sources it is
> mainly the latter which is expected to increase with increasing displacement
> of the object. The magnitude of the errors due to this source will be highly
> variable across the brain, and depend also on the strength of your magnet.
> Even for very small movements these effects may be quite large in especially
> the temporal lobes and the orbitofrontal cortex. It is therfore not easy to
> give a general blanket endorsment of movement up to any one limit.
> My, cautious, recommendation would be to include the realignment parameters
> as confounds in the design matrix as soon as you have any appreciable
> movement (>0.5mm). That way you will always err on the side of caution. Be
> aware though that this may potentially decrease your senitivity to "true"
> activations.
> 
> > Thanks in advance!
> > :> Daniel Weissman
> 
>                                                                      Good
> luck Jesper
> 
> 
> 


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