Hi Julia,
> We did not perform grand mean scaling so far.
> Is it common to do this, if one has data from different scanning days?
I completly agree with Stephen that sometimes, grand mean scaling could indeed help in those situations, also sometimes when the signal - for what ever reasons - is not stable over time.
Here just some other thoughts:
Have you tried to analyse A2 alone? That should give you a normal results, at least when there is nothing wrong with the second measurment. So, perhaps, as another alternative, you can do your between conditions comparisons on the second level, if you don't get it to work on the first level.
What about condition A1? Does that condition looks normal?
What about the high-pass filter? Are you perhaps filtering away your main effects in A2? Then this should happen in A1 as well.
Karsten
>
> Stephen J. Fromm schrieb:
> > On Tue, 13 May 2008 09:12:55 +0200, Julia Weiler <[log in to unmask]>
> > wrote:
> >
> >
> >> Dear SPM experts,
> >>
> >> I am a new user of fMRI and are experiencing some unclarities concerning
> >> my data interpretation.
> >> Our task required scanning the same subjects twice on two different days
> >> using the same task.
> >> We conducted three conditions (A,B,C) on the first day and one condition
> >> (A2) on the second day.
> >> A and A2 were basically the same. The scanner and scanning parameters
> >> were the same for the two days.
> >>
> >> When extracting the time courses using MarsBaR, we experience the
> >> following strange phenomenon:
> >>
> >> Conditions A,B,and C of the first day usually show similar time courses.
> >> However, compared to these
> >> conditions, condition A2 stays always close to zero.
> >>
> >> Hence, significant activations in a contrast A2 > B for instance, seem
> >> to be due to deactivation in B
> >> rather than activation in A2. For the reverse contrast, B>A2, however,
> >> there seems to be activation
> >> in B and no signal changes in A2.
> >>
> >> This is the same for a large number of ROIs. The time course for the
> >> condition of the second day is
> >> always close to zero with respect to the other conditions (although
> >> condition A2 of the second day
> >> was identical to condition A of the first day).
> >>
> >> Is it necessary to somehow normalize the data if I want to compare
> >> sessions acquired on different days?
> >> Or could I be making any other mistake?
> >>
> >
> > Did you perform grand mean scaling?
> >
> >
> >> Any suggestions would be very much appreciated.
> >>
> >> Regards
> >> Julia
> >>
> >> --
> >> Julia Weiler
> >>
> >> Ruhr-University Bochum
> >> Inst. of Cognitive Neuroscience
> >> Department of Neuropsychology
> >> GAFO 05/606
> >> Phone: +49-234-3223574
> >> Fax: +49-234-3214622
> >> =========================================================================
> >>
>
> --
> Julia Weiler
>
> Ruhr-University Bochum
> Inst. of Cognitive Neuroscience
> Department of Neuropsychology
> GAFO 05/606
> Phone: +49-234-3223574
> Fax: +49-234-3214622
>
--
-------------------------------------------------------------
Karsten Specht, PhD
Department of Biological and Medical Psychology
& National Competence Centre for functional MRI
University of Bergen
Jonas Lies vei 91
5009 Bergen
Norway
Tel.: +47-555-86279
Fax: +47-555-89872
[log in to unmask]
http://fmri.uib.no/
|
