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:
[log in to unmask]" target="_blank">http:[log in to unmask]
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
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