I ran the program exactly in a way that you suggested me, just for one subject, but still it is not correct. My goal is showing that activated areas have increased FA value during 5 time points in this subject.
The activation areas have different colors (red,orange, yellow and creamy yellow), only the pure yellow ones show increased FA value during 5 time points. Would you please help me to find out what still is wrong in my GLM that I can not get the correct result.
The GLM desing was as follow:
EV1 EV2
intecept Slope
1 1 -4.4
1 1 -3.4
1 1 -1.4
1 1 1.6
1 1 7.6
if I understand correctly your concern is that for some implicated areas you don't see a "linear increase", i.e. not every point (in time) is greater than the one preceding it? I can see why you would see this as an unwanted behavior, but it is nevertheless consistent with a linear model. Let us say you have five points and that the first four points have identical FA values and that the fifth point has a much larger FA. If you now fit a straight line to these points you will find a positive slope. It will no be a "perfect fit", but it will be positive.
There isn't really an easy way to pre-select the areas where each time point has a greater value than the previous. The best is probably to do precisely what you have done, to plot time courses for your different areas.
I hope this clarified it?
Jesper
Contrast EV1 EV2
Subject 1 0 1
Cheers,
Gelareh
On Tue, Jul 20, 2010 at 5:35 PM, Jesper Andersson
<[log in to unmask]> wrote:
Hi again,
Your assumption is correct and your GLM looks reasonable.
As I am very far away from mathematics, for my understanding would it be possible to explain for me from where below numbers: -4.4, -3.4, -1.4, 1.6 and 7.6 come from? I can see -3.4 to the end comes from -4.4+ month differences but from where -4.4 come?
these numbers are simply [0 1 3 6 12] - mean([0 1 3 6 12])
And another question: If I desing this, the the contrast would be as below?
Contrast EV1 EV2 EV3 EV4
Subject 1 1 0 0 0
Subject2 0 1 0 0
these contrasts look at the the intercepts for the two subjects. If you want to look at e.g. the slope (i.e. the changes over time) for subject 1 you should use [0 0 1 0]
Good luck Jesper
--
Gelareh Ahmadi, MD
PhD Candidate
Neuroimaging Group
Howard Florey Institute
University of Melbourne
VIC, Australia, 3010
ph +61 3 8344 1904
fax +61 3 9347 0446
