It doesn't really matter that in each session you don't have the same
number of stimuli per condition as long as across your 6 sessions you
end up with equal numbers (will work as well if not but it's not as good
because variance estimation can be biased). As for the different number
of conditions you can weight your contrast accordingly. Simply create 6
sessions in SPM and your 2 or 3 conditions in each sessions. Let say
your design runs like this:
Session 1 A B C
Session 2 A B
Session 3 B C
Session 4 A B C
Session 5 A B C
Session 6 A B
= 5*A 6*B 4*C
= 30/6*A 36/6*B 26/6*C (I choose to have 6 as denominator because you
have 6 sessions)
Let say you want to test A - B then use a contrast [30/36 -1 0 30/36
-1 0 30/36 -1 0 30/36 -1 0 30/36 -1 0 30/36 -1 0]
How do I end up with 30/36 and -1 --> 1/6 * 30/6 = 30/36 and -1/6 * 36/6
= -1 (I use 1/6 for each session since you have 6 sessions)
The sum doesn't end up to 1 but the contrast should still be valid ..
(at least using a simple design on my machine with unbalanced design and
manual checking the contrast is valid - check in SPM I think it is ok)
> Dear SPMers
> I am trying run first level analysis on fMRI data in SPM8.
> The experiment design involves 6 runs/sessions, each run/session consists of 8 stimuli (48 stimuli in total).
> There are 3 experimental conditions spread out equally across stimuli i.e. There are 16 stimuli of each condition in total.
> Because the stimuli were presented at random and are event-related, I am now running into a problem because in some of the runs of 8 stimuli, there happens to be only 2 of the conditions presented and not 3. In these cases, I'm not sure what to input in the 'onsets' tab of first level model specification for the 3rd condition.
> Is there a way around this problem. I'd be very appreciative of some help.
> Kind regards
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