Dear Volkmar,
I removed the first and last 4 volumes of each session and made all the
analysis as if it was a single session. It's somehow a simplification,
but it allows me to compute T-test on each of my parametric modulator
and to avoid F-test.
I also entered the realignment parameters so i think it somehow models
differences between sessions (movements were more pronounced between
sessions).
Thanks for you help.
Volkmar Glauche wrote:
> Dear Mathieu,
>
> a very brief answer: You should remove the first few scans from each
> session. Also, you should treat the sessions separately in realignment,
> slice timing (if applicable) and statistics. If you would treat your
> data as coming from one single session in the model, SPM would not be
> able to model e.g. different mean intensities and you would introduce
> errors in estimation of temporal autocorrelation and high-pass
> filtering.
>
> Hope this helps,
>
> Volkmar
>
> Am Mittwoch, den 05.11.2008, 19:39 +0100 schrieb Mathieu d'Acremont:
>> Dear SPM users,
>>
>> I'm analyzing fMRI data with SPM8b. Data were recorded during 4
>> functional sessions and 1 structural session:
>>
>> - session 1, functional, 408 vol.
>> - session 2, functional, 408 vol.
>> - session 3, structural, 192 vol.
>> - session 4, functional, 408 vol.
>> - session 5, functional, 408 vol.
>>
>> In each functional session, 4 volumes were recorded before the beginning
>> of the task (vol 1-4) and 4 after the end of the task (vol 405-408). So
>> only 400 volumes in each functional session are interesting for model
>> estimation (vol 5-404).
>>
>> For model estimation, i want to treat all volumes as if they were coming
>> from a unique session (because it is basically the same task in the 4
>> functional sessions and i need to simplify the model). I will also use
>> the realignment corrections computed during the preprocessing as
>> regressors in the model.
>>
>> I have 2 questions concerning the preprocessing:
>>
>> - should i define 4 sessions for the realignment or make as if data were
>> coming from a unique session?
>>
>> - which volumes should i delete during the preprocessing (e.g. vol 1-4
>> of the first session or of all 4 sessions, what about vol 405-408)?
>>
>>
>> Cheers,
>>
--
Mathieu d'Acremont, Ph.D.
Chief Scientist [log in to unmask]
http://people.epfl.ch/mathieu.dacremont
Phone/Fax: +4121 693 00 91/20
Swiss Federal Institute of Technology (EPFL)
Laboratory for Decision-Making under Uncertainty
Odyssea 3 18
Station 5
CH-1015 Lausanne
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