Based on statements in the SPM manual that Gaussian filters (and
smoothness) combine by Pythagoras’ rule, then...
You could use the following formula (in theory):
sqrt(FWHXfmri^2-FWHMvbm^2)=FWHMfilter
In practice, it doesn't seem to work out though for single images.
However, it seems like it would be better to match the smoothness of
residuls of your second-level models and not the first level models if
you want to match the smoothness of the spatial extents you will
actually compare.
Best Regards, Donald McLaren
=================
D.G. McLaren
University of Wisconsin - Madison
Neuroscience Training Program
Office: (608) 520-0586
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On Mon, Aug 16, 2010 at 3:14 PM, Gold, Brian T <[log in to unmask]> wrote:
> I calculated it from the residual fMRI time series. The fMRI data were
> analyzed in AFNI. In AFNI, you can get the residual time series during
> 3dDeconvolve by adding the option: -errts. The effective smoothness for x, y
> and z are then computed from the residual time series using 3dFWHMx. Then
> it’s just a matter of averaging the effective smoothness FWHM numbers across
> x, y, z directions, and then across subjects.
>
>
>
> Any ideas?
>
>
>
> Thanks.
>
>
>
> brian
>
>
>
> Brian T. Gold, Ph.D.
> Associate Professor
> Anatomy and Neurobiology
> MN 214 Medical Sciences Center,
> University of Kentucky,
>
> 800 Rose Street,
> Lexington, KY, 40536-0298
> email: [log in to unmask]
> office: 859-323-4813
> fax: 859-257-6700
>
>
>
> From: [log in to unmask] [mailto:[log in to unmask]] On
> Behalf Of Michael T Rubens
> Sent: Monday, August 16, 2010 3:15 PM
> To: Gold, Brian T
> Cc: [log in to unmask]
> Subject: Re: [SPM] effective smoothness
>
>
>
> How did you calculate effective smoothness?
>
> -Michael
>
> On Mon, Aug 16, 2010 at 11:22 AM, Brian Gold <[log in to unmask]> wrote:
>
> Hello,
>
> I am conducting a combined fMRI-VBM study. I want to compare the relative
> spatial extents of between-group differences in fMRI and VBM data sets. I
> thus want the effective smoothness of the functional and structural data
> sets to be as close as possible. I have computed the effective smoothness of
> my fMRI data to be 7.6 mm FWHM (averaged across x,y,z directions).
>
> What would be the simplest way in SPM 5 that I can determine the size of the
> FWHM smoothing kernel that, when applied to my VBM data, will result in the
> same effective smoothness as my fMRI data?
>
> Thanks in advance,
>
> Brian
>
>
> --
> Research Associate
> Gazzaley Lab
> Department of Neurology
> University of California, San Francisco
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