Hi folks -
I have a simple experiment with a block type design (16 second blocks) in
which I show four types of images plus periods of fixation, 4 times in a
scan. I then do another scan of the same conditions, with a different block
ordering.
I analyzed each scan individually using four EV's and creating a contrast for
each image type, plus some contrasts I am interested in. I got a lot of very
high z values in my contrasts (like as high as 13) which were consistent in
both scans, so I thought that combining the scans would give me a good boost
in power and reduction in noise.
However, when I combine the two runs in a higher level analysis (by selecting
both lower level feat directories as inputs and selecting all my contrasts),
the resulting maps are consistent with what I saw before, but the max z
values are now down around 2-2.5! This is true both for the contrasts of
single EV's (i.e. 1 for one EV and 0 for the rest in the low level analysis)
and for contrasts between EV's (i.e. 1 -1, or in one case, 3 -1 -1 -1).
Am I combining these two runs incorrectly? Or is it expected for the z values
to go down so drastically? here is my higher level GLM setup (its super
simple):
Number of EV's: 1
Number of groups: 1
Group EV1
Inp 1 1 1
Inp 2 1 1
Contrasts 1
EV1
C1 Combine 1
This is, of course, run on each of the lower level contrasts. I had expected
an "average" of my two runs, which is basically what I get, but with much
reduced z scores. Should I be including an EV for "run" or something to soak
of variance between the runs?
thanks,
Ed
--
Ed Vessel
New York University [log in to unmask]
Center for Neural Science
4 Washington Place, Suite 809 http://cns.nyu.edu/~vessel
New York, NY 10003
(212) 998-3908
