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.sch...@unige.ch> From:
gr...@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> >> > >
>> >> > _______________________________________________> > Freesurfer mailing
list> > Freesurfer@nmr.mgh.harvard.edu> >
https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer> > -- > Douglas N.
Greve, Ph.D.> MGH-NMR Center> gr...@nmr.mgh.harvard.edu> Phone Number:
617-724-2358> Fax: 617-726-7422> > Bugs:
surfer.nmr.mgh.harvard.edu/fswiki/BugReporting> FileDrop:
https://gate.nmr.mgh.harvard.edu/filedrop2>
www.nmr.mgh.harvard.edu/facility/filedrop/index.html> Outgoing:
ftp://surfer.nmr.mgh.harvard.edu/transfer/outgoing/flat/greve/> >
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