Hi,
On 10 Dec 2008, at 18:03, Shih-Wei Wu wrote:
> Thanks a lot.
> Just to further clarify:
> 1. In the case of seeing if there is an effect of each parametric
> regressor, and say that I have 3 parametric regressors and set up
> the contrasts --
> contrast 1: (1 0 0)
> contrast 2: (0 1 0)
> contrast 3: (0 0 1)
>
> rescaling or normalizing the value of each parametric regressor
> should not matter here because z is the normalized score of the
> parameter estimate.
>
> But rescaling/normalization is important when I want to compare
> different regressors. Say that I want to compare the parameter
> estimate of regressor 1 and 2 and therefore set up a contrast (1 -1
> 0).
>
> Is that right?
That's all correct - scaling isn't an issue in single-EV contrasts,
but can be for more-than-one-EV contrasts.
> 2. When you say that the EVs should be of height 1, do you mean that
> the maximum height should be of 1? Should I rescale them so that
> they all have the range [-1,1]?
Normally the raw EV waveforms (pre-convolution, e.g. as input via
either custom timings format), whether event-related or block, go from
0 (baseline) to 1 (activation) and so have a range 0:1.
Cheers.
>
> thanks
> shihwei
>
> On Wed, Dec 10, 2008 at 4:10 AM, Steve Smith <[log in to unmask]>
> wrote:
> Hi - yes this is important otherwise you can't easily interpret
> contrasts between conditions. The normal way is just to make sure
> that the EVs are of height 1 as input to FEAT, i.e. before
> convolution, etc., and then this is all comparable across EVs.
>
> Cheers.
>
>
>
> On 9 Dec 2008, at 01:42, shih-Wei Wu wrote:
>
> Dear all,
>
> A question with regard to scaling of parametric regressors.
>
> I have 3 parametric regressors that occupy different time points
> during a trial. The range of
> values between the regressors differ. One regressor has range
> approx. [-1,1] with mean at
> 0, the second regressor has range [-3,3], and the third regressor
> [-5,5].
>
> What would be the correct way to re-scale them so I don't have to
> worry about scaling
> problems when running the GLM?
>
> thanks!
> shihwei
>
>
>
> ---------------------------------------------------------------------------
> Stephen M. Smith, Professor of Biomedical Engineering
> Associate Director, Oxford University FMRIB Centre
>
> FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
> +44 (0) 1865 222726 (fax 222717)
> [log in to unmask] http://www.fmrib.ox.ac.uk/~steve
> ---------------------------------------------------------------------------
>
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Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve
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