Yes - my tool uses an FIR model by default
cheers
rp
On Dec 15, 2005, at 5:09 PM, Matthew Brett wrote:
> Hi,
>
> On 12/15/05, Steven Lacey <[log in to unmask]> wrote:
>> I am more confused than I thought. The MarsBar FAQ on the
>> calculation of
>> percent signal change (psc) does not mention fitting FIR models. As I
>> understand it, psc is calculated by:
>>
>> 1) multiplying the basis function for a given event of interest
>> (which is
>> extracted from the SPM.mat object) by it's corresponding beta value
>> 2) finding the max height of the resulting estimated response to a
>> single
>> event
>> 3) dividing the max height by the beta for the intercept
>> 4) multiplying the ratio by 100
>
> Yes - that's right - except that the "max height" above is only one
> way you could choose to calculate how much the event trace went up.
> Another might be the average height from 4-8 seconds, for example.
> This is the 'window' method. Others are:
>
> 'max-min', 'abs max', 'abs max-min'
>
> as listed in:
>
> http://marsbar.sourceforge.net/doc-devel/latest/marsbar/@mardo/
> event_signal.html
>
> Going back to your original question, you were comparing Russ' toolbox
> estimate of percent signal change to that from marsbar. I don't know
> Russ' toolbox very well, but I wonder if he is using an FIR model to
> estimate the signal change - whereas marsbar, by default, will not do
> this. In the default case, marsbar is using the basis function you
> used in your SPM model, and (in fact) the abs-max method of event
> trace height, so there is no windowing being used in marsbar. If Russ
> is using an FIR model for percent signal by default then that would be
> an obvious source of difference between the toolboxes.
>
> Best,
>
> Matthew
>
---
Russell A. Poldrack, Ph.d.
Assistant Professor
UCLA Department of Psychology
Franz Hall, Box 951563
Los Angeles, CA 90095-1563
phone: 310-794-1224
fax: 310-206-5895
email: [log in to unmask]
web: www.poldracklab.org
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