Dear Christy:
My understanding is that the smoothness estimates are derived from the residual
images not the original BOLD images. Thus due to differences in variances
between subjects the actual smoothness may vary considerably. Smoothness is
therefore related to the smoothing kernel you use, the more you smooth the
smoother the residual images will be, but the relationship is not absolute.
Having said this the amount of variation is more than I usually see. In a
previous email from 1999, Karl suggested 3 possible explanations
i) The T2* data were indeed acquired with very large voxels and slice
thickness.
(I'm guessing that all your subjects were acquired in the same way. So this is
not an explanation.)
ii) A spatial smoothing has already been applied to the raw data.
(My bet is you would know this, but it's worth checking.)
iii) There are severe spatially coherent confounds in your data that
have not been properly modelled. These can include drifts and global
effects.
(Perhaps there is something odd about the dataset. You might look at the images
with Matthew Brett's tsdiffana tool.)
I'm sure one of the spm stats folks can give a more detailed explanation.
Regards,
Darren
Quoting Christy Marshuetz <[log in to unmask]>:
> Dear SPM,
>
> I have an elementary smoothing question that I couldn't find an answer
> to in the archives. I have 4 participants, all of whom started with
> voxel sizes that had been resampled to 4 x 4 x 4 mm during realignment.
> I applied a smoothing kernal of 12 mm FWHM and ran models on each
> participant's data. When I look at the results, at the bottom of the
> results page, each participant has a different estimated smoothness.
>
> Participant 1: estimated smoothness (mm) = 11.9, 12.3, 11.5 mm = 3.0,
> 3.1, 2.9 (voxels).
>
> Participant 2: estimated smoothness (mm) = 19.0, 20.0, 17.5 mm = 4.8,
> 5.0, 4.4 (voxels)
>
> Participant 3: estimated smoothness (mm) = 14.0, 14.4, 13.1 mm = 3.5,
> 3.6, 3.3 (voxels)
>
> Participant 4: estimated smoothness (mm) = 15.5, 16.1, 14.1 (mm) = 3.9,
> 4.0, 3.5 (voxels).
>
> 1) Is this normal? If so, can I just ignore it?
>
> 2) If not normal, assuming that most of the time I didn't make any
> typographical errors, does anyone have any idea what might account for
> these differences?
>
> Thanks,
> Christy
>
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