Hi there
On Thu, 27 May 2004, Jack Grinband wrote:
> Hi All,
> I just wanted to confirm that FEAT only includes the non-derivative portion of the model in its
> hypothesis testing and that the parameter estimate associated with temporal derivative is not
> passed up to the group level. Is this correct?
>
The hypothesis tests and higher levels only consider the
relevant _contrasts_ of parameter estimates. Hence, if the derivative is
not included in any contrast (as is the default) then it will not be
inferred upon, or considered in higher level analyses.
> If the derivative portion explains a significant portion of the variance, you can get an artifactual
> decrease in the non-derivative parameter estimate? Is this right?
>
I don't believe so - I think that, so long as your EV starts and ends at
zero, then it is guaranteed to be orthogonal to its derivative.
\int x \dot{x} dx =1/2 x^2
so, if x(0) = 0 and x(end) = 0 then the inner product is zero.
There may be some sampling issues that I haven't thought of, but I think
you're basically ok to include a derivative without affecting your
original parameter estimate.
> I recently read a paper (Calhoun et al, 2004) which suggested using a hypothesis test that includes
> the parameter estimate for the derivative term. Does anyone have any opinion on this?
I haven't read this paper so I don't know exactly what is being suggested
here - hence no comment.
> Finally, is the PE for the derivative saved during the FEAT analysis? Where could I find it?
> thanks,
>
Yes - If your original EV is say EV3 , then its derivative will be EV4
hence the parameter estimate will be in the stats directory called
pe4.{hdr,img}
hope this helps
Tim
> jack
>
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Tim Behrens
Centre for Functional MRI of the Brain
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