Dear all:
I have a learning fMRI experiment, although there are previous message
on the similar topic, I am not sure whether what I'm doing is correct. I
would appreciate any help.
To be focused, I simplify the experiment as below:
One type of task, block design with 4 blocks alternatived with rest. The
prediction is a linear increasement or decreasement with blocks.
There are two possible methods:
Method one (getting learning effect after modeling):
Model each block as a "condition". Then test increasement with contrast
[-3 -1 1 3] or test decreasement with [3 1 -1 -3].
Q1: Is there difference between contrasts [-3 -1 1 3] and [-1 -0.5 0.5 1]?
Method two (getting learning effect while modeling)
Model all the 4 blocks as one "condition", and another parametric
regressor with [1 2 3 4] for increasement or [4 3 2 1] for decreasement.
Then test this parametric regressor by assign value 1 to it in contrast.
Q2: Is there difference between contrasts [1 2 3 4], [2 4 6 8] or [-3 -1
1 3]?
Q3: I am not sure whether the learning effect is increasement or
decreasement, but they may both be possible, or each one for different
brain regions. Should I add both of the two parametric regressors, [1 2
3 4] and [4 3 2 1], in one model? Or I must define two models for the
two parametric regressors separately?
Q4: I am using SPM5. It seems that there are two options for parametric
regressors, "Time Modulation" and "Parametric Modulation", which one
should I use? In addition, what values should I assign to the option,
e.g, just a vector [1 2 3 4] here?
Q5: There are some discussion about whether the above two methods are
equivalent. I am not a statistical expert, but for me, method one is
flexible and would save much time, after getting the beta value of the 4
blocks, I can do many tests using different contrasts, such as quadratic
trend. While for method two, changing a parametric regressor means
re-estimate the model, given the slow speed of spm(matlab), I can't
image how long it would take to find a suitable trend.
So, my question is, are both of the two methods acceptable by the
literature?
Thanks
Xiang
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