Dear all,
Karl's email that Robert is referring to also makes a suggestion at
ensuring (in a relative sense of the word) the validity of SVC with VBM by
making an explicit stationariness assumption in relation to the smoothness
of the regions undergoing SVC relative to the average smoothness of the
image (which should be achievable if grey matter partitions are smoothed
with 8-12mm). Provided that the smoothness of two hypothesised ROIs that
we applied SVC to (using p height and not extent statistic) was the same as
the average smoothness of the images (we used 12mm smoothing kernel), we
are relatively 'safe' in assuming the validity of our SVC statistics.
Would people agree?
many thanks for your help.
best wishes
Elena
At 13:20 21/02/2005 -0500, Robert McClure wrote:
>Dear Elena and SPMers:
>
>This question may make me unpopular, since it addresses the
>validity of using a SVC/mask with VBM, but I am going to ask it
>anyways...
>
>A number of groups have been applying SVC to VBM in their
>published studies. I understand that, at least in SPM99,
>non-stationariness of smoothing could make the SVC approach invalid
>(see message below).
>
>John, will you comment?
>
>_______________________________________________________________
>
>Date: Wed, 9 Jan 2002 13:04:17 GMT
> Reply-To: Karl Friston <[log in to unmask]>
> Sender: "SPM (Statistical Parametric Mapping)"
><[log in to unmask]>
> From: Karl Friston <[log in to unmask]>
> Subject: Re: svc in vbm
> Comments: To: [log in to unmask]
>
> Dear Christopher,
>
> John is right in the sense that SPM99 uses a volume measure that
> assumes the smoothness is stationary, The 'statistical' volume is
>resel
> (resolution elements) per voxel times the number of voxels. In SPM99
> the average resels per voxel over the search volume is used If the
> Small Volume in the SVC is small it may under-estimate or
>over-estimate
> the true statistical volume, if the local smoothnes is less than or
> more than the average. This is not a problem unless there is
> substantial nonstationariness in the smoothness. For VBM this may be
> the case.
>
> The way to fix this would be to use the resels per voxel estimate in
> the small volume (from the RPV.img). However, SPM99 does not use a
> local estimate. The simplest thing to do is to qualify your inference
> by making the stationariness assumption explicit and say the inference
> are only valid if the averge smoothness in the small volume is roughly
> the same as over the entire search volume. This can only be assured if
> the Gray matter partitions have been smoothed sufficiently (e.g.
>8-12mm
> FHWM).
>
> Note that these comments apply to the p value based on height (not
> spatial extent).
>
> I hope this helps,
>
> Karl
>
> ----- Begin Included Message -----
>
> From [log in to unmask] Wed Jan 9 11:05:44 2002
> References:
><[log in to unmask]>
> Date: Wed, 9 Jan 2002 11:08:56 +0000
> Reply-To: John Ashburner <[log in to unmask]>
> From: John Ashburner <[log in to unmask]>
> Organization: FIL
> Subject: Re: svc in vbm
> Comments: To: Christopher Summerfield
><[log in to unmask]>
> To: [log in to unmask]
> In-Reply-To:
><[log in to unmask]>
>
> The extent statistic is definately a problem because of the
>non-stationary smoothness
> of the residuals. Smoother regions are likely to produce bigger
>blobs. I would imagine
> that this non-stationarity would also have negative consequences for
>the validity of SVC
> for VBM data as it has been implemented in SPM99. Less smooth regions
>contain more
> resolution elements than average, so more independent t-tests are done
>and there is
> more chance of getting a false positive result (in these less smooth
>regions).
>
> Perhaps someone else can comment.
>
> Best regards,
> -John
>
> On Tuesday 08 January 2002 21:56, Christopher Summerfield wrote:
> > hi spm
> >
> > I seem to remember reading somewhere in the list a mail which said
>that
> > results using a small volume correction in voxel based morphometry
>may not
> > be reliable - is this the case? if so why? can the procedure be
>altered to
> > permit svc?
> >
> > chris summerfield
> > psychology
> > columbia university
>
> --
> Dr John Ashburner.
> Functional Imaging Lab., 12 Queen Square, London WC1N 3BG, UK.
> tel: +44 (0)20 78337491 or +44 (0)20 78373611 x4381
> fax: +44 (0)20 78131420 http://www.fil.ion.ucl.ac.uk/~john
>
>Robert K. McClure MD, Assistant Professor
>Department of Psychiatry, School of Medicine
>University of North Carolina at Chapel Hill
>Address: Campus Box #7160
>Chapel Hill, NC, 27510-7160, USA
>E-mail: [log in to unmask]
>Office phone: 919-843-6629
>
>
>
------------------------------------------------------------
Ms Elena Antonova
Post Doctoral Research Worker
PO78 Department of Psychology
Institute of Psychiatry
King's College London
De Crespigney Park
Denmark Hill
London SE5 8AF
UK
tel: +44 (0) 207 919 3048
fax: +44 (0) 207 919 2116
email: [log in to unmask]
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