Hi - thanks for your query.
Well, the simplest "bottom line" is that this is what happens if you don't
have strong activation. You could try using more liberal thresholds, but
you should probably also check that the analysis has indeed done what you
hoped.
One thing to check is that the paradigm is exactly right for your data -
one way of learning about the data without needing to specify the design
is to use MELODIC (ICA) software in FSL to see what's "in" the data.
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 Fri, 24 Aug 2001, Qingyan Guan wrote:
> Hi
>
> I had exactly same problem. I'm sorry I didn't get the point from your
> discussion. So what's the solution? Thanks.
>
> Qingyan
> >From: Stephen Smith <[log in to unmask]>
> >Reply-To: FSL - FMRIB's Software Library <[log in to unmask]>
> >To: [log in to unmask]
> >Subject: Re: [FSL] report.html missing activation image
> >Date: Thu, 23 Aug 2001 18:54:53 +0100
> >
> >Hi.
> >
> > > I looked at zstat.img and indeed the values were very low. I did find
> >some
> > > spurious voxels with Z's larger than 2.3, (for example, the one
> >report.html
> > > shows a time plot for (Z=4)).
> >
> >Some voxels have passed the first part of the cluster-based thresholding
> >(eg above >2.3) but then the resulting clusters fail the probability test
> >(eg is P<0.01) because they are not large enough. If you had chosen
> >resel-based thresholding instead of cluster-based you may have found a
> >little activation, but in this case the required Z threshold (derived from
> >the voxel-level P which you set) will be greater than 2.3 probably.
> >
> > > Certainly the experiment is no good. But, just out of curiosity, why
> >were
> > > these voxels not pointed out, even when FEAT is set to no thresholding?
> >
> >you get the time plot for max Z but no colour overlay because no clusters
> >survived thresholding and no average timecourse for the same reason.
> >
> >when FEAT is set to no thresholding that automatically carries through to
> >no colour overlay because no "activation" has been defined.
> >
> >Thanks, Steve.
> QingYan [log in to unmask]
> (425)391-8251
>
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