Dear SPM experts,
we are conducting an event related fMRI study and are in need of
some help in data analysis using SPM97.
We visually presented a total of 41 stimuli for 3 sec each withan
interstimulus interval of 16 +/- 2 seconds (uniformlydistributed). After
applying 20 identical stimuli (event type 1),one single stimulus (event
type 2) was presented that dramaticallychanges the ongoing experiment.
After this single event another 20
identical stimuli were presented. In general, we are interested in the
diffences in activation between event type 1 and 3 only. Weare aware of
the limited number of events per event type - but this was already the
maximum the paradigm would allow.
30 slices were acquired with TR=4.02 sec in an interleaved fashion
(1,3,5, ... , 28, 30).
Now our questions:
1.) first, what are your experiences regarding correcting the different
aquisition times of the slices. We have tried the method of Mark
D'Exposito and Geoffrey Aguirre (c.f., spm mailbase, July 10 1998, Re:
Time lag and hrf-modeling, by Darren Gitelman). Are there any better
solutions available or being developed?
Second, is it correct that any interpolation across slices is
problematic (e.g. realignment, normalization) in multislice recordings
with TR>2sec in an event related study?
As a simple solution one might shift the time vector of the events to
the acquisition time of the individual slice. Has anyone implemented
such a solution in SPM?
2.) in relation to question 1.) a fourier set as basis function would to
our knowledge eliminate the need for correcting for different slice
acquisition times (at least if no interpolation across slices is
performed before). Unfortunately, we encountered problems in
understanding the design matrix and setting the contrasts.
Furthermore, how many functions would be appropriate and how long
should the window length be?
3.) we treated our event type 2 as confound using the following spm
options:
- "make inferences about some events (treat others as confounds)"
- "events of interest": 1 3
furthermore we treated the time x response interaction as confound
because we expect differences over time because of the repetitive
presentation of the identical stimuli. We are at this stage not
interested in this interaction. We apply this confound to events
1 and 3:
- "apply to events:" 1 3
4.) The predefined temporal smoothing of SPM is 4 sec FWHM. Is this
really appropriate? (c.f., spm mailbase, July 27 1998, Re: single
trial analyses, by Jean-Baptiste Poline).
A (late) happy new year to everybody and many thanks for any hints
regarding these questions.
Bruno Weber
Alfred Buck
Stefan Kneifel
PET Center &
Dept. of Neurology
UniversitaetsSpital Zuerich
CH-8091 Zuerich
Tel.: ++41 1 255 55 79
Fax.: ++41 1 255 44 29
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