Hello.
I recently started to use spm99.
Would you please help me in understanding the following points related
to "resel" and corrected probability?
I used fMRI 120-scan (60 for test condition and 60 for control
condition) data without smoothing.
When I did statistics for a simple contrast between the two conditions
and input uncorrected p of 0.000001 for height threshold,
the following parameters appeared at the bottom of a result page.
Height threshold: T=5.01, p=0.000(0.047 corrected)
...
Degree of freedom=[1.0, 115.0]
Smoothness FWHM = 3.2 3.1 12.0 {mm} = 1.5 1.4 1.2 {voxels}
Search volume: S=408686mm^3 = 8715 voxels = 363.0 resels
Voxel size: [2.2, 2.2, 9.8]mm [1 resel = 2.56 voxels]
I cannot understand,
(1) why the ratio, S / resel, is 363.0 (2nd line from the last), instead
of 3404 expected from the last line, and
(2) if corrected p (pc) of 0.047 was calculated from equations: pc=1-exp(-E{m}),
E{m}=(S/w3)(u/2pi)^2exp(-u^2/2), w=FWHM/sqrt(4ln2) which appeared in
K.J. Friston et al., NeuroImage 40, 223-235(1996),
and "resel" was defined as FWHMx FWHMy FWHMz,
then substitution of u=4.76(z corresponding to t of 5.01) leads to S / resel=1511.
Thus, there are three different values for S / resel: 363, 3404, and 1511.
When I applied normalizing and smoothing to this data, they became
1485.9, 1764, and 2144.
Were they calculated from three different definitions of "resel"?
Are there any references which included definitions of the corrected p
and the resels shown in the result page?
(3)Additional question: "the search volume", S, was 1192616mm^3 for the
normalized and smoothed data,
while that was 408686mm^3 as shown above for the data without
normalization and smoothing.
If S denotes brain region to be searched, why did they differ so much?
Sincerely,
fujimaki
--
Norio Fujimaki, Ph.D
Brain Function Research Group
Auditory and Visual Informatics Section
Communications Research Laboratory
588-2, Iwaoka, Iwaoka-cho, Nishi-ku, Kobe, Hyougo 651-2492, Japan
Tel:+81-78-969-2261
Fax:+81-78-969-2269
e-mail: [log in to unmask]
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|