hi All,
We have two conditions, block design. We are interested in changes over
time. We ran our task as four separate scans/sessions (ie. we
started and stopped the scanner four times) only in order to give the
subjects a little break.
If we specify four sessions, then parametric modulation with time happens
separately for each session - but we want to look at changes over the
entire task, not within each session.
Say we parametrically modulate only cond2. So the design matrix is like
this
cond1 cond2 cond2PM
One option is to change the file names and treat them as though they were
all collected in one session (post realignment).
If we do this
1) will our power be hurt because we have a lot of variance due to session
but no session column in the design matrix?
2) In addition, might we get false positives due to linear scanner
drift? If so, could we eliminate this problem by
parametricaly modulating cond1 as well and using 0 1 0 -1 as a contrast?
3) Could we decrease error variance by using a regressor with all 1s for
files in the first session, 2s for files in the 2nd session, 3's for files
in the 3rd session and 4's for files in the 4th session?
3) Would a better solution to decrease error variance attributable to
session be to add four conditions named sess1 sess2 sess3 sess4 and give
onset vectors for them corresponding to every file in in sess1, 2 etc
respectively?
4) are there any other problems with treating four sessions as though they
were one?
Thanks,
Joanna
Joanna Salidis, Ph.D
Psychology Department
Stanford University
Jordan Hall
Stanford, CA 94305
Phone: 650-725-0797
Fax: 650-725-5699
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