Dear Richard & SPM list,
At 11:36 03/04/00 +0100, you wrote:
>Dear Sylvain,
>
>>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):
>
>This would certainly be the normal way of building your design matrix.
>However, I am not sure why you have included so many rows of five zeros in
>your version of the design matrix. Imagining that you had presented each
>condition once, in the order which you gave them (and just for simplicity
>of writing out the design matrix, that there was only one scan per
>condition), the usual design matrix would look like this:
>
>>100001
>>010001
>>001001
>>000101
>>000011
>
I've read a lot these past 4 days and I've added 1 condition column (the
6th, just before the column of 1) wich correspond the 'rest' periods
(scanned between the active tasks discussed above) So that I can compare
directly each condition to the rest.
>>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) ?
>>4) Are there interactions between the 2 factors. for example :"Does the
>>difference of activation between a & b depend on the level of Factor 1?"
>
>So, returning to your first model, you can think of your design as a 2 x 2
>factorial design, plus an extra condition (Bc). Your question 1 is the
>main effect of 'A' vs 'B' (contrast 1 1 -1 -1 0 0), and its converse
>contrast 'B' vs 'A' (-1 -1 1 1 0 0). Question 2 is the main effect of 'a'
>vs 'b' (contrast 1 -1 1 -1 0 0) and its converse. Question 4 is the
>interaction of 'A-B' with 'a-b' (contrast 1 -1 -1 1 0 0) and its converse.
>Question 3 is a question 'outside' the 2 x 2 factorial design, which is the
>simple main effect of 'b' vs 'c' in the context of B (0 0 0 1 -1 0) and its
>converse.
>
Ok, but now I have some questions about the "Mask with other contrasts"
function in SPM99. I want
to find every voxel that respond only to the B level of Factor1. I think
that a way to do that is to mask the contrast
(0 1 0 1 0 -2 0) ('B' vs Rest) EXCLUSIVELY with (1 0 1 0 0 -2 0) (A vs
Rest). Is this way the good One ?
And again thanks to Richard for his Help!
Sylvain Clement
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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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