Hi,
On 27 Feb 2008, at 02:01, Greg Burgess wrote:
> I'm not Steve, but I might be able to help. The featquery webpage
> says:
>
> "If you select Convert PE/COPE values to %, any PE or COPE parameter
> estimate or contrast values will be converted to percentage signal
> change
> values before reporting. This is achieved by scaling the PE or COPE
> values
> by (100*) the peak-peak height of the regressor (or effective
> regressor in
> the case of COPEs) and then by dividing by mean_func (the mean over
> time of
> filtered_func_data)."
>
> Instead of figuring out PPheights and mean_func for your ROI, you can
> estimate that scaling factor and apply it to your varcope values.
>
> 1) Extract cope & varcope WITHOUT clicking "Convert PE/COPE values
> to %"
> 2) Extract cope_psc WITH clicking "Convert PE/COPE values to %"
> 3) varcope_psc = varcope * (cope_psc / cope)
That's nearly correct but you need to take the sqrt of both varcopes
in that equation.
> What I'd like to know is: is this just an estimate, or is this
> actually
> precise (assuming that you use the same peaks, masks, etc.)???
It's very close - it may be different in some measurements as
averaging variance across a mask will not be exactly the same as
averaging the sqrt(variance) - but it's close enough.
Cheers.
>
>
> Thanks,
> --Greg
>
> ____________________________________________________________________
> Greg Burgess, Ph.D.
> Research Associate, Institute of Cognitive Science
> University of Colorado - Boulder
> Phone: 303-735-5802
> Email: [log in to unmask]
>
> University of Colorado - Boulder
> UCB 594
> Boulder, CO 80309-0594
>
>
>
>
>
> On Tue, 26 Feb 2008 09:08:09 +0000, Yvonne Brehmer <[log in to unmask]
> > wrote:
>
>> Hi Steve,
>>
>> I checked your recent replies to the questions regarding error bars
>> in %
>> signal change.
>>
>> One easy way is to get the COPE values with and without the '%'
>> conversion,
>> and then apply that factor to the square root of the varcope.
>>
>> Even though you might see this as a pure repetition, I would very
>> much
>> appreciate if you could be more explicit in how to do this maybe
>> including
>> the syntax code to run the calculation in fslmaths if possible.
>>
>> Thanks for your help,
>> Yvonne
>> =
>> =
>> =
>> =====================================================================
>
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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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