[Marsbar-users] Percent signal change question

[Thomas S. <tst...@ne...>]

Hi,

I used marsbar to extract percent signal change values for all 4 
conditions of a block design experiment.
The experiment included 4 conditions, and all of them have been 
modelled, because there is no real
baseline condition in this experiment. ROIS have been defined as spheres 
around the maxima of the
RFX-activation clusters. Percent signal change values have been 
extracted for each subject and each condition.
These values have been averaged to give a group-percent-signal-change 
estimate for each experimental condition.
For some of the ROIs, all of these four values are negative, for other 
ROIs all of them are positive.
Now the question:
As I read in the marsbar documentation, all the values are scaled to the 
mean of the roi.
How can it be that the values do not sum to zero? The 4 conditions model 
the whole period of the experiment,
so I thought that they should sum to zero.

Best regards
Thomas Stephan

-- 
Dr. Thomas Stephan
Abteilung fuer funktionelle, strukturelle und molekulare Bildgebung
Zentrum fuer Sensomotorik
Klinikum Grosshadern der Universitaet Muenchen 
Marchioninistr. 23
81377 Muenchen

tst...@ne...
+49 089 / 7095-4819 (Fon)
+49 089 / 7095-4801 (Fax)