[Marsbar-users] comparability of data from different scanning sessions

[Julia W. <jul...@ru...>]

Dear MarsBaR experts,

I am a very 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?

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