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]
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>
> 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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