Dear Rik,
Thank you very much!
One more question regarding the model setup: in SPM99 I defined the 6
runs of my experiment as one session so that the onsets of run 2 didn't
start at 0 but after the onsets of run1.
In SPM2 this doesn't really work: I either get error messages or a
rather strange looking design matrix. When setting up a FIR model is
there any reason not too model each run as a separate session?
Best,
Simone
Rik Henson wrote:
>
>> I've been trying to obtain the time course of activation. First of
>> all I'm
>> not sure, if the length of my time bins should equal the TR (3 sec)
>> or if it
>> would be wiser to choose shorter bins of, lets say, 2 sec? Time bins
>> should
>> cover the period from 5 time points before to 5 time points after the
>> onset
>> of the condition so that the whole time course should cover 30sec.
>> Since SPM 2 no longer offers the distinction beween "event related" and
>> "epoch related" designs I'm not quite sure how to specify the design
>> if I do
>> not want to convolve my data with the canonical HRF. Which basis set
>> should
>> I choose for the model setup? FIR model? If so what about the window
>> length
>> (30?), order (10?) and duration (0?)?
>
>
> Yes, yes, yes! (if you want a bin size of 3s).
>
> The first FIR bin will start with the event onset, so if you want to
> start
> 5 timepoints before the "true" event, then subtract 15s (5 TRs) from your
> onset vectors (but note that SPM2 will not handle negative onsets less
> than -2 TRs, so you may have problems for the first event).
>
> There is no point having FIR bin sizes less than your TR unless you have
> achieved a higher effective sampling rate by jittering your event
> onsets with respect to the scan onsets, either randomly or by ensuring
> that your
> SOA is not a simple multiple of your TR. (So if you locked your event
> onsets to scan onsets, then there's no point having a bin size less
> than 3s
> in your case).
>
> HTH
> Rik
>
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