Hi,
There are certainly papers in which thickness is corrected using global mean thickness, and surface area is corrected using total surface area.  It is a valid approach, and one that I think is preferable to correcting by using a somewhat arbitrary adjustment to a non-like measure.

Re (3):  If you wish, you can compute your “global” covariate for surface area or thickness excluding the contribution of the region that you are testing.  Then, the covariate is for the “rest of the brain”, and not impacted by the contribution from the region that you are testing.

We have done both these things in the following:
http://www.ncbi.nlm.nih.gov/pubmed/20118463

cheers,
-MH

-- 
Michael Harms, Ph.D.
-----------------------------------------------------------
Conte Center for the Neuroscience of Mental Disorders
Washington University School of Medicine
Department of Psychiatry, Box 8134
660 South Euclid Ave. Tel: 314-747-6173
St. Louis, MO  63110 Email: mharms@wustl.edu

From: <freesurfer-bounces@nmr.mgh.harvard.edu> on behalf of maaike rive <r_maaike@hotmail.com>
Reply-To: Freesurfer <freesurfer@nmr.mgh.harvard.edu>
Date: Friday, May 6, 2016 at 3:53 AM
To: Freesurfer <freesurfer@nmr.mgh.harvard.edu>
Cc: "Marie.Schaer@unige.ch" <marie.schaer@unige.ch>
Subject: Re: [Freesurfer] does lGI needs correction for any global measure?

Hi Doug,

 

Thanks for the reply.

I'm still struggling with this issue though.

 

1. For instance, if one corrects surface area for ICV in the GLM, shouldn't the fact that this relationship is non-linear be taken into account? Since volume increases with r^3 and surface with r^2... Same goes for lGI, how is the relationship between lGI en ICV?

 

2. If I choose to correct for TSA, it seems natural to correct thickness for mean (or total) thickness and lGI for some global measure (is there such thing as mean or total lGI?) as well, or am I mistaken? I don't understand why one measure should be corrected for a global measure with the same units, whereas another should not.

 

3.  ICV is not influenced by the volumes of the different brain structures (it is not a summation of these volumes) so volume correction for ICV is unbiased. However TSA (or mean thickness, or "global lGI measure") is dependent of the area (or thickness or lGI) of each discrete brain structure, right? So for instance if one group has a smaller superior frontal area, total area will be smaller as well. Wouldn't correction remove part of the effect then?

 

Maaike

 

 

 


 

> To: freesurfer@nmr.mgh.harvard.edu; Marie.Schaer@unige.ch
> From: greve@nmr.mgh.harvard.edu
> Date: Thu, 5 May 2016 12:32:11 -0400
> Subject: Re: [Freesurfer] does lGI needs correction for any global measure?
>
>
>
> On 05/02/2016 07:39 AM, maaike rive wrote:
> > Dear freesurfer experts,
> >
> >
> >
> >
> > I have a question regarding correction of cortical measures or global
> > measures (ICV, total surface area). As I understand from literature
> > and the freesurfer wiki, a model for thickness should not be corrected
> > for ICV (since thickness does not scale with total brain volume), but
> > surface area should be corrected since is does scale with total brain
> > volume. Three questions:
> >
> >
> >
> >
> >
> > 1. should I use ICV or should I use total surface area (TSA) as
> > covariate in the model to correct surface area (by the way, average
> > ICV and TSA do not differ between the groups to compare )?
> >
> >
> Not sure. They will be different models. ICV should not change over the
> lifetime whereas total surface area will.
> >
> > 2. does lGI also need correction? does it scale with volume? If so, do
> > I need to correct for ICV or for TSA? Usually I see no correction but
> > some do use ICV...
> >
> >
> I don't know, perhaps Marie will weigh in
> >
> > 3. even if the average of certain measures (like ICV/TSA, or gender,
> > or age) does not differ between groups, should they be added to the
> > model? If I do not correct for gender, for example, my results change
> > and most papers do use age and gender as covariates if they compare
> > groups despite a lack of differences between groups...
> >
> >
> They can still reduce intersubject variance.
> doug
> >
> >
> >
> >
> > Thanks,
> >
> >
> >
> >
> >
> > Maaike
> >
> >
> >
> >
> > _______________________________________________
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> > Freesurfer@nmr.mgh.harvard.edu
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>
> --
> Douglas N. Greve, Ph.D.
> MGH-NMR Center
> greve@nmr.mgh.harvard.edu
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> Fax: 617-726-7422
>
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