Dear experts!
We are analyzing data from a bloc design fMRI study (2 condtitions A and
B one control condition C, all appearing within one run, two different
groups of subjects 1 and 2, two runs / subject).
We want to perform group comparisons on the second level. We are
interested in the three contrasts:
(A1-C1)>(A2-C2),
(B1-C1)>(B2-C2),
(A1-B1)>(A2-B2).
All of the contrast images on the first level look o.k. However some of
the contrasts on the second level (for within [one sample t-test] and
between [two sample t-test] group comparison), in particular those where
we expect very little activation, show significant activation in white
matter areas and areas that are not activated in first level analysis
(in spite of very good realignment parameters, we attribute this
activation to residual head motion).
our questions are:
1 could it be, that if very little variance is present in grey
matter, the significance of signal fluctuation in white matter
increases?
2 would it be appropriate to perform 'threshold masking' for between
group comparison? (and if yes how is this done?)
3 for contrast (A1-B1)>(A2-B2) would it be appropriate to include the
control conditions C1/C2 by performing second level analysis of 4
contrast images [(A1-C1)-(B1-C1)]>[(A2-C2)-(B2-C2)]?
Thanks very much in advance
Bernhard
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Dr. B. Haslinger
Neurologische Klinik
TU Muenchen
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