Not sure what you mean, in particular what processing you applied to
the probtrack outputs to get them into a common space.....?
On 29 May 2009, at 19:57, John Kuster wrote:
> Hello Steve,
>
> Thanks for your explanation. Could you please explain how this
> applies to
> probtrack output? We have used randomise to compare probtrack
> outputs from a
> ROI in subjects and controls and are unsure of how to correct in this
> case...
>
> Thank you!!
>
>
> ---------------------------- Original Message
> ----------------------------
>> Subject: Re: [FSL] stastical corrections?
>> From: "Steve Smith" <[log in to unmask]>
>> Date: Tue, May 19, 2009 6:09 am
>> To: [log in to unmask]
>> --------------------------------------------------------------------------
>>
>> Hi,
>>
>> There are various practical reasons why TBSS samples the data into
>> 1x1x1mm standard space, primarily because having the mean-FA skeleton
>> at this resolution works much better in practice than at lower
>> resolutions. We don't generally recommend doing a "VBM-style"
>> voxelwise analysis (i.e. using the first stage of the TBSS processing
>> only) for the reasons outlined in the original TBSS paper - however,
>> you're right, if we _did_, then working at 2x2x2mm would probably be
>> just as good.
>>
>> You're right that upsampling the data complicates the multiple
>> comparison correction - but at any given resolution, and taking into
>> account intrinsic data smoothness, it is important to get all these
>> issues taking into account. Hence both Gaussian random field theory
>> inference explicitly corrects for effective smoothness for you, and
>> permutation-based inference, with FWE thresholding, implicitly also
>> takes all these things into account, to give valid multiple
>> comparison
>> correction. If you take either of these approaches in general when
>> done correctly you are ok.
>>
>> Cheers.
>>
>
>
> On 5/18/09 1:32 PM, "John Kuster" <[log in to unmask]> wrote:
>
>> I apologize if this has already been posted to the list, but I did
>> not see
>> it come through successfully. Thanks!
>>
>>
>>
>> Dear FSLers,
>>
>> We have a question regarding correction for multiple comparisons in a
>> diffusion tensor imaging study:
>>
>> We have conducted a DTI study in which we have a priori areas of
>> interest
>> in our voxel-based analysis. We've pulled out the areas of interest
>> and
>> are only evaluating them initially, so the correction for multiple
>> comparisons should be based on the number of voxels considered, with
>> smoothing/clustering criteria taken into account.
>>
>> So far, this is all fine. However, the problem arises when I
>> realize that
>> the registrations that FSL uses to put DTI data into standardized
>> space
>> (using an MNI template), takes 2x2x2 data and registers it into a
>> 1x1x1
>> volume. The voxel-based contrast is done on the registered 1x1x1
>> images,
>> and thus, eight times the number of comparisons are made than would
>> have
>> been made on the original data. So the question is: How do we
>> correct for
>> multiple comparisons? It doesn't make sense to me to "increase"
>> resolution and the number of comparisons without actually adding real
>> information.
>>
>> Does anyone have any suggestions how to deal with this situation?
>>
>> Thank you!
>>
>>
>>
>>
>>
>>
>>
>>
>
>
>
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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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