Dear Will Penny, SPMers,
1) In order to compare the modulatory effects of 2 tasks (that were
perfomred in 2 sessions) I followed your suggestion (below) and joined the
2 sessions into 1 time-series in the GLM model, and then created a DCM
model with 2 input variables: TASK_1 and TASK_2.
When I come to compare the effect of the two tasks through the "contrast
of connections" option: I put 1 for effect Bij in the matrix of TASK_1
(then 'done') and -1 for effect Bij in the matrix of TASK_2 (then 'done').
However, from the display of the results it seems that this is comparing
Bij of Task 1 to other B's of Task 1, and Bij of task 2 to other B's of
task 2, rather than Bij in task 1 to Bij in task 2. (This is because the
other B's of task 1 are black rather than grey, and the other B's of task
2 are white rather than grey.)
So my question is: what is this contrast comparing, and what is the
correct way to compare Bij in task 1 to Bij in task2.
2. What is the way to read the graph that results from this "contrast of
connections" option? what do the axes mean, and why are there 2/3 peaks?
Thanks a lot
Tali Bitan
northwestern University, IL
On Fri, 20 Feb 2004 15:02:21 +0000, Will Penny <[log in to unmask]>
wrote:
>What I suggest is you join the sessions together. If you put another
regressor
>in your original design matrix with a load of 1's at the border this will
>nicely stitch the time series together - any nasty transition effects
will be
>soaked up by this regressor. You can then treat the two sessions as a
single
>time series.
>
>You will then be in a position to create your modulatory input variable
>which is -1 for the first part of the time series and +1 for the second
(or
>0 and 1). You can then test for the strength of the modulatory effect as
>outlined above.
>
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