| 1) This may be a really idiotic question, but how does one view the
| uncorrected t-statistic images? I'm assuming that viewing the t-statistic
| images for a given contrast using the default values: "corrected height
| threshold = no", "threshold {T or p value} = 0.001", and "extent threshold
| {voxels} = 0" still applies a correction that is based on the the
| smoothness estimates and consequently the number of resels.
This displays the raw uncorrected t statistics that are more significant
than p<0.001. There is no correction for the number of resels when you
dont specify a corrected height threshold.
Another way of displaying the statistic images is to use <Display> or
<Check reg>.
|
| 2) The size of the bounding box strongly influences the smoothness
| estimates, which I assume are then used to generate the significance maps.
|
| Normalization to the SPM99 Default bounding box results in the following
| values:
|
| VOL.
| S: 21242
| R: [2 43.5423 508.3141 1.2528e+03]
| FWHM: [2.6842 2.5119 2.1405]
|
| whereas normalization to a bounding box which is the same size as the
| original volume results in
|
| VOL.
| S: 23350
| R: [1 22.7378 110.1009 125.8692]
| FWHM: [5.9930 5.6235 4.7065]
|
| In both cases, the voxel size is 3.75 x 3.75 x 5
|
| The larger bounding box has a smoothness estimate that is twice as large
| as the smaller bounding box, and an order of magnitude fewer resels. I
| assume that this is why the t statistic images differ so much.
|
| It is somewhat puzzling that the smoothness estimate would depend so
| strongly on the bounding box, unless if the smoothness estimate is derived
| from a particular spatial frequency bin, which would correspond to a lower
| spatial frequency given a larger bounding box.
I have no idea why the smoothness estimates are so different. Were there any
other differences in the analyses of the data. Less good model fits often
produce more residual smoothness. Is there anything bizarre in the normalised
data? For example, if you have two sets of writing normalised images running
at the same time, then you can have problems.
Regards,
-John
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|