Dear Cyril,

Thanks for your quick reply.
Each session starts and ends with a 'baseline' condition to prevent problems due to the scanner starting, and also the first two volumes are discarded. Do you think that there is still a problem with that? (well, conditions order was different between sessions, but if there is a problem with that there is no question I will need to reanalyse 15 peoples data . . .).

Thanks so much,
Christina



On 12/5/06, cyril pernet <[log in to unmask]> wrote:
Hi Christina:

>Dear SPMers,
>
>I am new to SPM and I would appreciate any help on the following question.
>I have a four session design. Each session contains all the conditions I
>want to compare later on through contrasts. When presented, the sessions
>follow each other and there is a very short break between sessions
>presentation (30sec approx.).Participants stay still and they only get out
>of the scanner after they have completed all the sessions. I have analysed
>the data as one single session (instead of 4) as this made contrast
>definition easie ....  I am now unsure about how correct this is. Do I
>violate any assumptions? Is it a common procedure to regard multiple
>sessions as one? Is this &#8216;eligible&#8217;?
>
>
I think that's a bad idea - each time you stop start the scanner you
have variations. Therefore you should use 4 sessions in your design,
each one having its' own cste. Contrasts are not more complicated, if
you enter the conditions the same way you should be able to use exactly
the same contrasts.

Best
Cyril