Dear All,
I am trying to desing an event related fMRI experiment with 4 event types.
Before I embarked on this project I searched the mailing list for the
necessary practical advice, and also read most of the pertinent literature.
Although I think I know what the model that I want to use, there still
remain some practical ambuiguities that I think I need to resolve.
I hope that some of you will read my description below and tell me whether
the model that I have created is accurate ( within limits!)
There are 4 event types. Each is 8000 ms long.
In SPM 99 I specified a stochastic model as follows.
TR = 3.9 sec
scans per session = 90
4 conditions
null events +
SOA =2 ( this is something unclear to me. How does one select the SOA? is
there a rule of the thumb?)
relative freq = 1 1 1 1 1
occurrence probability - modulated
basis function = hrf with time derivatives ( because I am interested in t-
images)
no interactions and no user specified repressors.
I will have about 10 sessions.
These are the questions.
1.Is this a generally OK model?
2.How does one select the SOA?
3.once the scanning is done, how does one select the correct scans? Is there
a trick? Or do you just count them?
4.When I look at the temporal vectors that SPM generates I realize that for
some sessions there are 38 events and for others 34. Is this correct?
5.Is it possible to look at the time vector in matlab space in terms of
scans and not in terms of seconds?
6. What other practical advice would you offer a non expert user of SPM for
event related analysis?
Thank you,
Sincerely,
/Priyantha Herath
|