Dear Jerry, Alex & everyone:
I'd like to point out that a similar query, also by Jerry Allison was posed
(and answered by JB Poline) some months ago in the SPM list. See:
http://www.mailbase.ac.uk/lists/spm/2000-04/0172.html
Briefly, the answer consisted of a recommendation to check the residuals
(ResMS.img), because such unexpectedly high values of smoothness are most
likely due to high residual variance, and this can happen for instance when
sessions were not scaled to the same value.
Best regards,
-Alexandre
>
> Dear Jerry,
>
> Did you spatially normalize the scans before running your statistical
analysis?
> If so, there is a smoothing step in normalization that could account for the
> increased smoothness found in your final images.
>
> Cheers.
>
> Alex.
>
> > Date: Wed, 18 Oct 2000 16:24:52 -0500
> > Subject: Smoothing Criteria Revisited.
> > From: "L. Stephen Miller" <[log in to unmask]>
> > To: [log in to unmask]
> > Cc: [log in to unmask]
> > X-List: [log in to unmask]
> > X-Unsub: To leave, send text 'leave spm' to [log in to unmask]
> > X-List-Unsubscribe: <mailto:[log in to unmask]>
> >
> >
> > In March of 2000
> > (http://www.mailbase.ac.uk/lists/spm/2000-
> > 03/0205.html),
> > Karl offered the following relative to smoothing.
> >
> > "The smoothness should be 2-3 times the voxel size of
> > the data that
> > enters the estimation procedure (i.e. after spatial
> > normalization).
> > One can therefor subsample the data to smaller voxels
> > and then smooth with
> > a kernel that approximates the original voxel size. It is
> > important to
> > note that the smoothness is the post hoc smoothness of
> > the residual fields
> > (given in the SPM table footnotes). This smoothness
> > may be much greater
> > than the size of the smoothing kernel."
> >
> >
> > I recently analyzed fMRI data that has native voxel
> > dimensions of
> > 3.75x3.75x5mm.
> >
> > After realign & coregister, voxel dimensions were still
> > 3.75x3.75x5mm.
> >
> > Without applying any smoothing via spm_smooth.m,
> > SPM reports the
> > smoothness as (given in the SPM table footnotes) as:
> >
> > FWHM 8.6x8.6x10mm = 2.3x2.3x2 voxels
> >
> > Thus it seems that without applying any smoothing via
> > spm_smooth.m, my
> > data meet the smoothness criteria.
> >
> > Is this to be expected?
> >
> > My usual approach is to apply a smoothing kernel of
> > twice the native voxel
> > size (e.g.7.5x7.5x10) regardless of the smoothness
> > reported by SPM , but
> > if I understand Karls criteria, no smoothing need be
> > applied to this data
> > to accomplish the desired smoothness. Actually, if I apply a filter kernel
> > of 7.5x7.5x10mm to the data via spm_smooth.m, SPM
> > reports the smoothness
> > as: FWHM 48.9x52.8x60.2mm=13x14.1x12 voxels!!!!!!!
> >
> > So, is it best to process the data initially without applying
> > any
> > smoothing, and then based upon the smoothness
> > reported by SPM decide
> > whether and how much smoothing to apply via
> > spm_smooth.m?
> >
> > Jerry Allison, Ph.D.
> > Medical College of Georgia
> > L. Stephen Miller, Ph.D.
> > Associate Professor
> > Department of Psychology
> > The University of Georgia
> > Athens, GA 30602-3013 USA
> >
> > TEL: (706) 542-1173
> > FAX: (706) 542-8048
> > email: [log in to unmask]
>
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