Hi again,
And thanks! - Well, in this case I used slice time correction and therefore the microtime resolution (specified at 1:st level) should be set as the number of slices. This parameter is set for the entire 1:st level model, and hence the problem. I guess a way would be to skip slice time correction, but I would hope there is another way to come around this.
Best
Sandra
> On 19 Jan 2016, at 16:03, <[log in to unmask]> <[log in to unmask]> wrote:
>
> It sounds like a lot of your problem comes from the notion that you cannot
> model the sessions together because of the number of slices issue. But that
> does not follow at all. Your data gets normalized and everyone is moved into
> common space, presumably before the statistics are run. This should remove
> that issue. I have dealt with data where our MRI tech dynamically changes
> slice numbers to get whole brain coverage (not my choice, not my study), and
> we never had any problems.
>
> Just make sure you are careful with preprocessing steps and it should be
> fine.
>
> Sorry I have never used flexible factorial for group differences so no help
> there.
>
>
>
> -----Original Message-----
> From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On
> Behalf Of Sandra Tamm
> Sent: January-19-16 5:33 AM
> To: [log in to unmask]
> Subject: [SPM] Modelling 2 1:st level contrasts per subject
>
> Dear SPM-experts,
>
> I have a question regarding an experimental task. We have 2 groups of
> subjects (different age) scanned at 2 separate occasions one month apart
> with 2 different sleep conditions before the scanning. In the fMRI task, we
> have some conditions with 1 specific contrasts of interest. In the beginning
> of data collection we changed the scanner sequence and the number of slices
> and therefore, some of the subjects have different number (45 or 46) of
> slices on their 2 scanning sessions. This, I believe, makes it impossible to
> model both sessions together at 1:st level.
>
> I made separate 1:st level models for the 2 sessions with corresponding
> contrasts of interest. Now I want to perform 2:nd level analyses and this is
> where I come in to trouble.
>
> First of all I want to see the overall effect of the task conditions. The
> obvious way would to perform a one sample t test with contrasts from first
> level. However, since I have 2 contrasts for each subject (from 2 separate
> sessions) I guess I cannot put all the 1:st level contrasts in to one test,
> because the contrasts are not independent (since every subject has 2
> contrasts). Is there a simple way to handle this? (i.e. modelling the
> subjects in a reasonable way).
>
> Secondly, I am interested in both effects of the sleep conditions (i.e.
> between sessions), the effect of age group, as well as interaction between
> age and sleep condition. The effect of sleep is easy (paired t tests), as
> well as the interaction (I used a flexible factorial design), but I read in
> earlier emails that it is not valid to test for group differences in the
> flexible factorial. Is this true? If yes, is there another way?
>
> Best regards
> Sandra
>
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