hi tim,jack,
The parameter estimates are orthogonal to one another, and the nonderive
parameter is often used as an estimate of the hemodynamic amplitude..but it
is correlated with the latency of the fmri response. that is...if the fmri
response is shifted, but doesn't change in amplitude at all, the parameter
estimate for the non-derivative ev will decrease...introducing a latency
bias. The point of that paper is just that this estimate is biased by
latency...and that using both parameters to estimate the fitted amplitude
removes this bias.
Hope this helps,
vince
>> 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
>>
>
>--
>-------------------------------------------------------------------------------
>Tim Behrens
>Centre for Functional MRI of the Brain
>The John Radcliffe Hospital
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>Oxford University
>Work 01865 222782
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