To me, it makes much more sense to use mean cortical thickness as a covariate for thickness-based analyses.

cheers, Mike H.

Hi Mehul,

the MNI group had an abstract showing that thickness didn't need eTIV correction at HBM a number of years ago, and it has been our experience as well.

cheers, Bruce

On Sat, 31 Oct 2009, Mehul Sampat wrote:

...

Hi Folks,

If am comparing the mean thickness for certain gyri (pre-central, post-central) in a controls versus patients. I obtain the thickness measurements from lh.aparc.a2009s.stats and rh.aparc.a2009s.stats I was wondering if I need to normalize these thickness measurement with the estimated total intracranial volume (eTIV) ?

Or has anyone reported that the these are independent of eTIV ? If so then as I understand I would only need to account for age and gender in any subsequent analysis using GLM.

Thanks Mehul

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Well, if doing a thickness analysis, then mean cortical thickness seems the "natural" covariate to use, if you want to answer the question whether the thickness difference in a region is "beyond" any difference that would be predicted based on mean cortical thickness. This is especially relevant if mean cortical thickness differs between groups, as I've seen in some samples...

cheers, -MH

On Sun, 2009-11-01 at 19:45 -0800, Mehul Sampat wrote:

Thank you for your suggestions Bruce and Mike.

Mike, could you please provide the motivation behind your suggestion of using mean cortical thickness as a covariate ?

Let's consider this is the context of a simple example. Let's say we compare the mean thickness of the pre-central gyrus in patients versus controls. Does the biology suggest that even after one accounts for age and gender, there are natural variations in the gyrus thickness measurements ( that is, some people would just happen to have larger thickness measurements ) ?

If this is the case, then one could normalize the thickness of the individual gyrus measurement (pre-central gyrus in this example) with the mean cortical thickness. However, in most of the papers I have only seen age and gender used as co-variates.

Thanks Mehul

On Sun, Nov 1, 2009 at 4:44 PM, Michael Harms mharms@conte.wustl.edu wrote:

    To me, it makes much more sense to use mean cortical thickness
    as a
    covariate for thickness-based analyses.

    cheers,
    Mike H.


    > Hi Mehul,
    >
    > the MNI group had an abstract showing that thickness didn't
    need eTIV
    > correction at HBM a number of years ago, and it has been our
    experience
    > as well.
    >
    > cheers,
    > Bruce
    >
    > On Sat, 31 Oct 2009, Mehul Sampat wrote:
    >
    >> Hi Folks,
    >>
    >> If am comparing the mean thickness for certain gyri (pre-
    central,
    >> post-central) in a controls versus patients.
    >> I obtain the thickness measurements from
    lh.aparc.a2009s.stats and
    >> rh.aparc.a2009s.stats
    >> I was wondering if I need to normalize these thickness
    measurement
    >> with the estimated
    >> total intracranial volume (eTIV) ?
    >>
    >> Or has anyone reported that the these are independent of
    eTIV ? If so
    >> then
    >> as I understand I would only
    >> need to account for age and gender in any subsequent
    analysis using GLM.
    >>
    >> Thanks
    >> Mehul
    >>

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