Thank you very much for your reply. I am posting the conversation to the list again since someone else might have a similar question.
So, if I understood you correctly, you suggest to use the fixation periods as a new baseline? The problem here is similar to what I have with the 0-back baseline though - if the fixation activation is different (for ex if the general arousal of the 2 groups is different), there is then no way to demonstrate that, correct? Would comparing the betas values for a given condition between groups give any additional information (f ex betas for 0back (grA) vs betas for 0back(grB)?
Thank you very much!
----- Original Message -----
From: "Donald MCLAREN" <[log in to unmask]>
To: "Olga Prilipko" <[log in to unmask]>
Sent: Monday, May 11, 2009 8:22:27 PM GMT +01:00 Amsterdam / Berlin / Bern / Rome / Stockholm / Vienna
Subject: Re: [SPM] Baseline differences between groups
1- is an implicit comparison against fixation, so in that sense it
does remove the intersubject variability. Of course, if you find
differences here, are those due to difference in 1- or the fixation
state. Since BOLD is relative, one can not asses group differences in
terms of one-state or another; only that one or both states of the
subtraction are different between groups.
On Mon, May 11, 2009 at 9:51 AM, Olga Prilipko <[log in to unmask]> wrote:
> Dear SPMers,
> I would very much appreciate help on the following problem:
> I am comparing 2 groups of subjects on an n-back task. Both groups performed 0-, 1-, 2- and 3-back conditions. I use 0-back as baseline and compare the groups with 2 -sample t-test for 1>0(grA) vs 1>0 (grB), 2>0(grA) vs 2>0(grB) etc
> However, I suspect that the baseline condition activation(0-back) is very different between the 2 groups, which would make my 2-sample ttests difficult to interpret..
> I did a "direct" comparison 1-back(grA) vs 1-back(gr B) etc, but of course it does not remove the intersubject/session variability and it is thus difficult to know which differences are relevant.
> Does anyone have an idea for testing for differences between the baselines?
> Thank you very much!
Best Regards, Donald McLaren
University of Wisconsin - Madison
Neuroscience Training Program
Office: (608) 265-9672
Lab: (608) 256-1901 ext 12914
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