Dear Rik,
thank you! I've just tried the FIR model and have a probably very naive
question regarding it: if the first FIR bin starts 5 timepoints before
the onset of the "true" event (timepoint 0?) and the last one starts 5
timepoints after the true event I have 10 timepoints altogether but do
not have a timepoint for the true event?
Shouldn't I have 11 timepoints if I want the onset of the "true" event
included and therefore specify 11 for "order" when doing the model setup?
Best,
Simone
Rik Henson wrote:
>
>> 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.
>
>
> I would not advise doing this. 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 no longer represent a continuous timeseries, and several aspects of SPM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.
>
>> 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?
>
>
> There is no reason not to - model each run as a separate session.
>
> Rik
>
>
>>>> 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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