Dear SPM group,
I'm planning an fMRI experiment and I have problem in defining my design
matrix for performing statistics with SPM99.
I will use a 'box-car' stimulations paradigm alternating between Rest &
Task periods.
I am studying the effects (& may be the interactions) of 2 factors:
* Factor1 with 2 levels:
A
B
* Factor2 with 3 possible levels (a, b & c)
BUT the designed is not well balanced (not sure that it's the right word):
* For the level A of factor1 I can have level a & b of factor 2
* For the level B of factor 1 I can have levels a, b & c of factor 2.
Therefore, I get 5 kind of task period ( (Aa), (Ab), (Ba), (Bb), (Bc)) that
are presented 3 times each (random order).
I can imagine 2 way of defining my design matrix :
1) Tell SPM that there are 5 conditions with each condition being a kind of
task period ( (Aa), (Ab), (Ba), (Bb), (Bc)). That gives me a matrix like
that (if each cond are presented only once):
00000
10000
00000
01000
00000
00100
00000
00010
00000
00001
00000
2) Tell SPM that there are 5 conditions but with 2 condition defining the
levels of factor1 and 3 conditions defining the levels of factor2. I then
get the following matrix:
AABB
ababc
---------
00000
10100
00000
10010
00000
01100
00000
01010
00000
01001
00000
The question I'd like to answer are :
1) Does the 2 levels A & B of Factor1 Activate different regions ( I think
that I have exclude the c level of factor2 to answer) ?
2) Does the levels a & b of Factor2 activate different regions ?
3) Does the levels b & c of Factor2 activate different regions (I think I
have to exclude the A level of factor1) ?
3) Are there interactions between the 2 factors. for example :"Does the
difference of activation between a & b depend on the level of Factor 1?"
What is the best way to analyse my data?
I've already perform analysis with SPM99 on a previous quite simple
experiment with only two conditions with no problem but here, I've
difficulties to choose my design even after spending time to read the SPM
course notes.
Thanks in advance
Sylvain Clément, PhD student in Bordeaux (FRANCE)
------------------
Sylvain Clément
B.P. 63, UMR CNRS 5543
146 rue Léo Saignat
33076 Bordeaux Cedex, FRANCE
Tel: 05 57 57 16 51
Fax: 05 56 90 14 21
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