Dear SPM Experts,
Here are just another two spm beginners ...
We have a simple eventrelated Design, as follows:
Task:
Every 4'th fMRI scan (TR = 3000 ms, scan duration 1650
ms) the probant have to make a decision with respect
to a presented stimuli.
This was done 125 times.
Control:
The same than Task, but no decission was made.
In a first Analysis we want to look only on the
task-run, just asking witch aereas are activated
(independly from the decission made).
We have done preprocessing:
realign, time and slice, normalize, smoothing
and specifyed our fMRI model as:
-> specify and estimate a model
-> No. of sessions 1
-> interscan interval 3
-> trials 1
-> SOA fixed, 4, 0
-> parametric modulation none
-> events
-> hrf with time derivation
-> filter gaus 4, hrf 32
-> F contrast
-> contrast 1 0 0
When we examine the Result, we get for a uncorrected p
Value of 0.001 very large activated areas.
Now decreasing the p value at first shrinks the
activated areas, but then, for a long range (1e-5 ...
1e-15) the changes are very small, and then, by a step
of 1e-17 (from 0.6e-16 to 0.5e-16) all activations
vanihses.
This is in particular suprising because the last map
where we get activatios has a wide range of diffrent
grayscaled (p valued) voxels (and still quite large
activated areas).
Is this a numeric problem ? May we have to change
numeric resolution in matlab ?, and: are the small p
Values realistic ?
Thanks for help in advance !
Felix and Sebastian
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___________________________________________________________
Dipl. Phys. Sebastian Thees
Universitätsklinikum (Charite)
der Humboldt Universität zu Berlin
Neurologische Klinik
Schumann Straße 20/21
10117 Berlin, Tel: 030 - 2802 5115
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