Stephan Hamann wrote:
>
> I created a stochastic event-related design with null events (using a
> randomization program I've written), acquired some data, and now am having
> a difficulty analyzing the data with SPM99.
>
> When I specify "stochastic" under the fMRI models: specify design option,
> it asks me whether there is a null event type, which is good, but then it
> asks for a single SOA, which doesn't make sense because I need to enter a
> vector of onsets for each condition.
>
> If I take the non-stochastic/deterministic option, it asks me for vectors
> of onsets (which is good) but it never asks whether there are null events
> (bad) and I am concerned that by telling SPM99 this is a non-stochastic
> model that the analysis will not proceed correctly.
>
> Perhaps the problem is that SPM99 wants to design the experiment for me
> (by default) when I select "stochastic" and I need to somehow circumvent
> its help and hand-enter the vectors of onsets?
Stephan -
The question "stochastic design? yes/no" is a bit confusing, and you
are correct at the end that you should have pressed "no".
You would only answer "yes" to this question if you want SPM to
create a design matrix for you, before you've run the experiment
(you could then take the onset timings implicit in the resulting
design matrix directly into your stimulus presentation software).
Though your design is "stochastic", you have already run the
experiment (already have the appropriate onsets) and therefore
should press "no" in response to the question. You will then be
asked whether the SOA is fixed or variable, to which you would
press "variable" (because your design is stochastic), and then
you will be prompted for onset times (in scans) for each event-type.
You will not be prompted for null events because they are not
modelled differently from the baseline.
We should really rephrase the "stochastic design?" question, because
you're not the first person to be confused by it!
Rik
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