Dear Alan & David,
> To try to avoid mood induction being confounded by order effects, the
> experimental design counterbalances the order of the three mood induction
> tasks across all control subjects and Bipolar subjects.
>
> while for each individual subject we would not be able to distinguish
> effects of mood induction from those of session order, we wonder whether
it
> would be possible to examine the main effects of mood induction over all
> subjects using a group analysis.
>
> is it possible and valid using spm99 to identify the main effects of mood
> induction for each group (control and Bipolar).
No I am afraid not. Even using a random-effects (i.e. second-level
analysis of contrasts from each session) the main effect of mood is
completely removed by the constant or CD term of the High-pass
filtering (it would in fact be removed even if you omitted filtering).
This is because the differences in mean fMRI signal from session to
session have so many sources of variance experimental effects are
generally obscured.
You could certainly try it by creating session-specific contrast images
where the contrast isolates to constant term in the design matrix for
each session (and entering these contrasts into a two sample t test)
but I suspect you will not get very good results. To study enduring
state or set-related brain responses of this sort you would normally
use PET.
With best wishes - Karl
|