Dear spm users,
we have performed a simple motor task in 13different subjects, each
formed by 18x3 scans (6 baseline, 6 A-condition, 6 B-condition, 3 repeat).
In order to obtain a "mean image of activation", we used a design matrix
with 702 scans (54x13 snr*.img).
The analysis steps is mentioned as follows:
1. fmri models: specify and estimate a model.
2. mum of session: 13
3. select scan for session 1-13: the snr*.img of 13 subjects.
4. interscan interval (secs): 5 (TR=5000ms)
5. are conditions are replicated? Yes
6. are timing/parameters the same? Yes
7. number of conditions or trials: 3
8. name for condition/ trials 1-3: trial1.. trial2.. trial3..
9. SOA: fixed
10. SOA(scans) for trial1: 18
11. times to 1st trial(scans):0
12. SOA(scans) for trial2: 18
13. times to 1st trial(scans):6
14. SOA(scans) for trial3: 18
15. times to 1st trial(scans):12
16. parametric modulation: none
17. are these trials: epochs: fixed response (box-car)
18. convolve with hrf: yes
19. add temporal derivatives: no
20. epoch length for trial1-3: 6.. 6.. 6
21. interactions among trials (Volterra) no
22. user specified regressors: 0
23. remove global effect: scale
24. high-pass filter?: specify
25. low-pass filter: hrf
26. model intrinsic correlations?: none
27. setup trial-specific F-contrast?: yes
Is it conceptually correct this kind of group analysis? If it is a group
analysis, what is the formal name of this method?
And to see the RESULT, I set the contrast as these:
A vs baseline (-1 1 0 -1 1 0 -1 1 0 -1 1 0 -1 1 0 -1 1 0-1 1 0 -1 1 0 -1 1
0 -1 1 0 -1 1 0 -1 1 0 -1 1 0)
B vs baseline (-1 0 1 -1 0 1 -1 0 1 -1 0 1 -1 0 1 -1 0 1 -1 0 1 -1 0 1 -1
0 1 -1 0 1 -1 0 1 -1 0 1 -1 0 1)
To see the (A > baseline)>(B > baseline), I used exclusive masking for (A >
baseline) with (B > baseline).
Then, we select
Correct height threshold: no (Does it mean I didn’t do the correction for
multiple comparisons?)
Threshold (T or p value): 0.001
Then I got a “fine” result.
In fact, I also tried to select
Correct height threshold: yes
Correct p value: 0.05
But the activations regions can’t be found.
I think i don't understand very well the thresholds and the p-values
(corrected, uncorrected)
And are there some mistakes in the above process?
Thank you very much.
Thanks,
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