Hi - thanks for the email.
After reg to standard space _all_ subjects must contain valid (non-zero
variance at the first-level) data at a voxel for group stats to proceed at
that voxel.
Do you get this apparent artefact in fixed _and_ random effects analysis?
If you go back to the first-level analysis, how many subjects appear to
show this artefact?
I would be more than happy to look at your results - if you would like to
tar up the group feat directory (and even the first-level ones as well)
and stick them on a web site I can have a good look.
Thanks, Steve.
Stephen M. Smith
Head of Image Analysis, FMRIB
Oxford University Centre for Functional MRI of the Brain
John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve
On Wed, 31 Oct 2001, Robert Coghill wrote:
> Hi All,
> In group analyses of a large number of series, we have found some
> activation around the edges of the dorsal frontal cortex that looks a
> bit artifactual. We've done quite a bit of work to ensure that all of
> functional to highres to standard alignments are satisfactory and are
> now wondering if these potential artifacts are due to missing data at
> the edges of the brain in some individuals. Does a group FEAT analysis
> use something like a common elements mask to exclude voxels in regions
> with less than 100% of fMRI volumes contributing or does it compensate
> statistically for such missing data?
> Thanks,
> Bob
> --
> Robert C. Coghill, Ph.D.
> Department of Neurobiology and Anatomy [log in to unmask]
> Wake Forest University School of Medicine phone: (336) 716-4284
> Winston-Salem, NC 27157-1010 fax: (336) 716-4534
>
> http://www.wfubmc.edu/nba/faculty/coghill/coghill.html
>
|