Just to put my mind at rest, I ran a few analysis on the same data, with
different sized bounding boxes (the "template" and "spm95" bounding boxes).
When no global correction was used, the uncorrected t statistics were
identical. When global correction was done, the uncorrected t statistics
differed ever so slightly. This is because the globals obtained depend
on the bounding box used, as different regions of the head are used to
compute them.
The smoothness estimates were very similar in all cases, but these should
have no influence on the computed t statistics. The smoothness estimates
will differ slightly because the bigger bounding box will cover a different
region of the head, and also because of a slightly different fit to the
model because of different global estimates. Smoothness is computed from
those voxels that the stats are done on (see mask.img).
Regards,
-John
| Having now established (as per the interchange quoted above) that we are
| talking about the same thing when we talk about displaying uncorrected
| t-statistic images via the SPM99 interface, the point remains that the
| uncorrected t-statistic images differ as a function of the bounding that
| is specified during normalization.
|
| The difference in the number of significant voxels in the t-statistic
| images plotted as described above appears to be directly related to the
| number of resels, and the number of resels is a function of the size of
| the bounding box. When we generated the normalized images, the only
| parameter we changed was the bounding box size. We created the two sets
| of normalized images and performed the analyses one after another, so I
| don't believe we could have run into the problem of writing two
| different sets of normalized images at the same time as you suggested
| might be the problem below. Forgetting for the moment about trying to
| normalize into a bounding box that was the same size as our original data,
| we used two of the SPM99 bounding box options: the SPM99 default, and
| option #4. Both give differing estimates of the smoothness, and
| consequently the number of resels and significant voxels:
|
| Normalized Smaller bounding box (SPM99 Default)
| 3948 significant voxels
| VOL =
| S: 21242
| R: [2 43.5423 508.3141 1.2528e+03]
| FWHM: [2.6842 2.5119 2.1405]
|
| Bounding box option 4
| 2545 significant voxels
| VOL =
| S: 21788
| R: [2 37.2294 383.8368 832.4568]
| FWHM: [3.1065 2.9114 2.4696]
|
| Is the smoothness estimate from which the number of resels is
| derived computed over the entire image or only those parts of
| the image that exceed some threshold?
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