Dear Alan & David,
> The aim of the experiment is to examine the effects of experimental
> mood induction on verbal fluency activations, in bipolar patients and
> normal controls.
>
> Each subject undergoes 3 mood induction tasks each lasting approx 6.5
> mins. Each mood induction task is followed by verbal fluency tasks
> during which an fmri time series is obtained. Each verbal fluency task
> consists of 30 sec epochs of each of three conditions Repition (A),
> Generation (B) and Rest(C), ordered as follows:
>
> C A B C C B A A B C C B A
>
> The first rest epoch is discarded prior to analysis.
>
> In analysing the data so far, to examine verbal fluency effects, and
> modulation of these by mood, we have used two cut off filters as
> follows:
>
> haemodynamic response function ( HRF)
>
> high frequency cut-off: 180 180 180
>
> the high frequency we presume is optimised depending on previous input
data
>
> we would be grateful for your help in the determination of the
> appropriate filter and contrasts to allow us to best examine the main
> effects of mood.
I am afraid you cannot assess the main effects of mood per se because
they are completely confounded with session. You can assess the mood x
condition interaction which is the aim of your study and would involve
modelling all the sessions in a session-separable design matrix
(enforced by SPM99) and using appropriate contrasts. These contrasts
can be assessed directly in terms of SPM{T} at the first level for each
subject (fixed-effect analsis) or entered into a one-sample T test at
the seond-level (random effects analyis). I would use a two sample T
test at the second level to assess group differences in the mood x
condition interaction contrasts.
e.g. for a deisgn matrix with the following efffects
subject 1 subject 2 ...
mood- mood+ mood- mood+ ...
A B C A B C A B C A B C ...
0 1 -1 0 1 -1 0 0 0 0 0 0 Main effect of fluency - subject
1
0 -1 1 0 1 -1 0 0 0 0 0 0 Mood x fluency (vs. rest) interaction - subject
1
0 0 0 0 0 0 0 -1 1 0 ........
I hope this helps - Karl
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