Hi Daniel and SPMers,
I believe that the SPM99 realignment is robust with respect to
correcting for large movements (e.g., between run repositioning by the
subjects) however, it has been our experience that this can sometimes be
substantial (greater than 10 mm or degrees, especially with patient
populations. Since we analyze the runs as separate sessions in statistics,
I prefer to realign them separately. Also, we (and others) have noticed
that there can be substantial differences in the global mean (across all
voxels within run) between sessions. For example, the image intensity for
one session could range from 6000-8000 (mean of 7500), while another run may
range from 8000-10,000 (mean of 9500). We can control this somewhat in
the reconstruction of our images, but these 'gain' differences do exist.
SPM realignment may be robust to this change in means across sessions, but I
preferred not to test this. We typically then realign within session,
create mean images, then normalize these mean images to the EPI template and
then apply these parameters to the rest of the images within the respective
session. Another tip I have implemented here is to reposition (using the
new display facility in spm99) all of the images within each run so that
they closely resemble the position of the EPI template. This really appears
to make the spatial normalization more robust.
Also, FYI, we are getting very similar results between spm99 and spm97,
however, the overall significance for spm99 is much higher than spm97.
Best regards,
Kent
______________________________
Kent A. Kiehl, Ph.D.
2255 Wesbrook Mall
Departments of Psychology and Psychiatry
University of British Columbia
Vancouver, B.C., Canada, V6T 2A1
http://www.psychiatry.ubc.ca/sz/nilab/personnel/kiehl/
Office 604-822-0777; Lab 604-822-7128; Fax: 604-822-7756
----- Original Message -----
From: Daniel Weissman <[log in to unmask]>
To: Kent Kiehl <[log in to unmask]>
Cc: spm <[log in to unmask]>
Sent: Tuesday, March 14, 2000 8:21 AM
Subject: Re: Number of Sessions per subject
> Hi Kent,
> Thanks for the advice about modelling sessions separately.
> Regarding the realignment procedure, I'm not sure I understand why it's
> better to realign each session separately compared with realigning the
> images as if they all came from a single long session. In either case,
> aren't all of the images just realigned to the first image acquired?
>
> Thanks,
> Daniel Weissman
>
>
>
> > Hi Daniel,
> > I would tend to enter each session separately and not to concatenate
> > into one long session per subject.
> > I can think of a few reasons for this, primarily b/c the event-related
> > response do not overlap across sessions. Using this procedure one can
still
> > create a subject specific contrast for random effects analyses. I would
use
> > the default settings for the high pass filter. Low pass filtering is a
> > little different story - I believe it is recommended to use the hrf, but
we
> > typically filter our data outside spm b/c we (and others) have noticed
high
> > frequency noise can be prevalent in some scanners. I also would realign
> > each of the sessions separately.
> > Other groups may have different opinions.
> >
> > Best regards,
> > Kent
> > ______________________________
> > Kent A. Kiehl, Ph.D.
> > 2255 Wesbrook Mall
> > Departments of Psychology and Psychiatry
> > University of British Columbia
> > Vancouver, B.C., Canada, V6T 2A1
> > http://www.psychiatry.ubc.ca/sz/nilab/personnel/kiehl/
> > Office 604-822-0777; Lab 604-822-7128; Fax: 604-822-7756
> >
> > ----- Original Message -----
> > From: Daniel Weissman <[log in to unmask]>
> > To: spm <[log in to unmask]>
> > Cc: <[log in to unmask]>
> > Sent: Monday, March 13, 2000 12:30 PM
> > Subject: Number of Sessions per subject
> >
> >
> > > Hi,
> > > I've just run an event-related fMRI study that in which I
> > > collected 8 "blocks" of data for each subject. Each block was a
6-minute
> > > long scan. I'm wondering whether it is better to enter each "block"
as a
> > > separate session in the analysis (i.e., 8 sessions per subject), or to
> > > "pool" all 8 blocks from each subject into one huge session (i.e., one
> > session
> > > per subject).
> > >
> > > In the discussion list, some have recommended "pooling"
> > > multiple sessions into one big session for each subject (9/22/98),
> > > but others have argued that such pooling might not be a good idea
> > > (10/15/99).
> > >
> > > I'd appreciate any advice and, if it is possible to perform the
> > > analysis in both ways, I'd be interested to know whether and how the
> > > high- and low-pass filters should be set in each case.
> > >
> > > Thanks,
> > > Daniel Weissman
> >
> >
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