Dear Karl, Tor, Rik, Karsten, and SPM community,
I have very much appreciated the recent posting. I can see that sampling
much below 0.5 Hz is not really worth it (see one of Rikīs mails). I
have not fully understood the use of t- and F-contrast in the case of
Volterra analysis though.
Karl has pointed out not to join weighings of A and A*A in one line of
an F-contrast (essentially, they are quite different and the
interpretation of such a contrast in not very straightforward) but to
use instead:
> A B A*A A*B B*B
> 1 0 0 0 0 to see which voxels respond to A, and
> 0 0 1 0 0
in a two trial type design. However, I am still dazzled about Torīs
other question:
> > Also, if I want to test the DIFFERENCE between A and B, I have to specify
> > a different kind of contrast?
>
> The way to test for a difference is to remove the commonality. This
> would be most simply effected by doing another analysis where you
> modelled A and B as one trial type and B (i.e. the difference) as
> another and then use the contrasts above. It is possible to work out
> the F contrast for your design (but I do not know how to - JB does
> though!).
Could JB or you, Karl, provide some further assistance to clarify?
Also, how about t-contrasting? Obviously,
> A B A*A A*B B*B
> 1 0 1 0 0 to see which voxels respond to A,
shouldnīt be used in a t-test either, right? Could I use
> A B A*A A*B B*B
> 1 0 0 0 0
AND
> A B A*A A*B B*B
> 0 0 1 0 0
and procede to a conjunction to assess which voxels respond to A in both
a linear and nonlinear way? Does that at all make sense? And how about
the voxels responding positively in either a linear or nonlinear way?
> A B A*A A*B B*B
> 1 0 0 0 0
> 0 0 1 0 0
is not really feasible for a t-contrast, is it?
I am also interested in a three trial type of design (A, B, C). How to
correctly contrast the difference between A + B and C?
A B C ...
1 1 -2...
or
0.5 0.5 -1...
How to deal with the interactions in such case?
A B C A*A A*B A*C B*B B*C C*C
Can non-orthogonality of as much as 0.79 between A and A*C be handeled
by SPM99?
Thanks a lot in advance for any feedback. Merry X-mas, I guess I am not
posting any more before the holidays... andreas
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