Re: [Marsbar-users] contrasts comparison

[Joseph P. <ph...@ho...>]

Hi Matthew,

Thank you very much for your reply.

The ROIs are different because they refer to different anatomical
structures of the same subject. They were defined based on the subject's an=
atomy, so I guess I am not running into problems of bias. I
would like to test if the average activation in structure 1 is greater
than in structure 2 for a single subject.=20

Is there a way of extracting the sum of squares (SS) of the contrast
in each ROI, and use this SS for comparing the average activation in
the two ROis (a two-sample t-test)?

Best regards,
Josue> Date: Mon, 2 Jul 2007 11:21:06 +0100> From: mat...@gm...>=
 To: ph...@ho...> Subject: Re: [Marsbar-users] contrasts comparison>=
 CC: mar...@li...> > Hi,> > > I am reading the marsb=
ar devel tutorial. At the end of the tutorial, two> > contrasts are compare=
d (p.30, "You can see that the contrast value =96 which> > is proportional =
to the change in signal for a single event =96 is greater for> > run 3 than=
 for run 1"). My question is: how can I test if those two> > contrasts are =
significantly different?> >> > I believe that one answer is to run a single=
 model with run1 and run3 and> > test the contrast [1 -1] ([Run1 Run3]).> >=
 Yes - that's right.> > > However, if the ROI for run1 is different than th=
e ROI for run3, how could I> > compare those two contrasts? In this case, I=
 wouldn't be able to test them> > inside the same model (suppose I would li=
ke to test if left amygadala> > activation is greater than right amygadala =
activation for a single patient).> > Well, for a single patient, that is a =
problem, because you'd expect> the variances and so on to be different betw=
een the two ROIs.  You> could get round this by creating a new time course =
for the two> sessions, two ROIs, in matlab for example (extract for session=
 1, save> to variable, extract to session 2, save to variable, concatenate,=
 then> import data from Data menu.> > Why are the ROIs different?  If you'v=
e selected the ROIs on the basis> of the activation for that session, you c=
an easily run into problems> of bias.> > > A second unrelated question is: =
For answering the question "does area A on> > average activate more for con=
dition 1 than condition 2", marsbar calculates> > a new summary time course=
 for each ROI and uses this time course to estimate> > the model. Would it =
be correct to calculate the mean of the contrast values> > for every voxel =
inside the ROI (calculated in spm con_*.img files) and use> > this value as=
 an estimate of the average activation in the ROI? How this> > procedure di=
ffers from marsbar method?> > It's very little different - it's only that, =
if you use marsbar for> the first level analyses, you can adapt the model c=
orrectly to the ROI> autocorrelation - but your suggestion is a reasonable =
shortcut...> > Matthew
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