Hi Nick,
Perhaps I'm missing something, but how do you code the non-linear
formula in STEP1 in terms of a contrast vector 'c'? That is, I don't
see how the variance equation in STEP2 can be the variance of the
variable computed in STEP1...
cheers,
Mike H.
On Tue, 2009-11-10 at 16:17 -0800, Nick Wymbs wrote:
> After some additional digging, I think I have the solution to my
> nonlinear question. Please correct me if there's any error!
> To reiterate, the formula that I used to run a main effects contrast
> with the main condition PE and it's temporal derivative is the
> following:
>
> STEP1: sign(PE1)*sqrt(PE1^2+PE2^2) .
>
> From here, the varcope must be generated in order to get the correct
> error term in the denominator. I didn't use prewhitening so there
> formula is quite simple:
>
> STEP 2: varcope == c' * inv(X' * X) * c
>
> Where c is the contrast vector and X is the full design matrix.
>
> When considering the contrast vector, both the main EV and temporal
> derivative EV receive an equal weight (1) and everything else is 0.
> Finally, in order to generate t-stats, the result from STEP 1 is
> divided by the square root from STEP 2 (error).
>
>
> Cheers
> Nick
>
>
>
> On Mon, Nov 9, 2009 at 4:28 PM, Nicholas Wymbs <[log in to unmask]>
> wrote:
> Hi FSLers
>
> I'm trying out a contrast that uses the Calhoun (2004) method
> of incorporating the temporal derivative using the formula:
> sign(PE1)*sqrt(PE1^2+PE2^2) where
>
> PE1 == standard EV fit
> PE2 == temporal dv of EV
>
> I then divided by the SEM of the residuals (res4d.nii.gz). The
> values do not look like t-stats, with the range being
> something like -250 to 250. Now I'm faced with the really
> basic question: how are the PEs converted to t's and z's? I
> know the conversion is supposed to be pretty straight-forward
> and it states on the feat page that 'To convert estimates of
> parameter estimates (PEs) into statistical maps, it is
> necessary to divide the actual PE value by the error in the
> estimate of this PE value', which is what I thought I did when
> dividing by the SEM of the res4d file. What's goin' on?
>
> Thanks
> Nick
>
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