On 12/5/06, PESENTI Mauro <[log in to unmask]> wrote:

LLN, le 5/12/06

Dear Christina,

Cyril is right: concatenation is not recommended. If the scanner
stopped and restarted between runs, yet you modelled your conditions
as a single session, then your data no longer represent a continuous
timeseries, and several aspects of SPM will be disrupted, eg.:

- highpass filtering,
- temporal autocorrelation estimation,
- grand-mean scaling,
- session-meaning.

(I quote Rik Henson, from this list).

Note that I once compared "concatenated runs + constant" to
"separated sessions" and the results were quite similar, though not
identical.

>I will need to reanalyse 15 peoples data . . .).

Hmmmm, what are 15 individual analyses in the life of a brain-mapper...?

Hope this helps,

Mauro

>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
><<mailto: [log in to unmask]>[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 ‘eligible’?
>>
>>
>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

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Mauro PESENTI
Research Associate, National Fund for Scientific Research (Belgium)
Unite de Neurosciences Cognitives
Departement de Psychologie
Universite Catholique de Louvain
Place Cardinal Mercier, 10
B-1348 Louvain-la-Neuve

tel.: +32 (0)10 47 88 22
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