Hi Jeff and all,
For normalization (i.e., divide the
measurement under study by some
global measurement), I would not argue
favourably, as this procedure can
bias the results in the opposite direction
if a global effect is present.
Instead, include it as a covariate is not
as harmful. My suggestion is,
when there is no clear approach about
using or not a global measurement
as a nuisance, the relationships between
the measurement under study,
the independent variable, and the putative
nuisance should be
calculated, and models with and without
the nuisance should be analysed
and presented. The discussion should
consider both analyses together,
and enough information should be presented
so that the final
interpretation is left to the reader.
Specifically for area, I suggest analysing
and presenting two models:
(1) without any global measurement and (2)
with global area as nuisance.
If brain volume (whichever way it is
measured) is to be considered a
potential nuisance for the disorder you
are analysing, it can be
included in the model #2 above, even given
that they are not orthogonal
to each other, and are related to global
area. Non-orthogonality
between the nuisance variables is not a
problem as it is when effects of
interest are involved.
Hope this helps!
All the best,
Anderson
On 23/03/12 11:29, Michael Harms wrote:
> Our reply to that is here
>
http://bjp.rcpsych.org/content/196/5/414.2.long
>
> which reminded me of other papers
that have also used a global thickness
> measure to covary for mean cortical
thickness and thereby "address whether
> any regional thickness differences
were in excess of global cortical
> thickness differences between groups"
-- see references [1,4] in our
> Reply.
>
> cheers,
> -MH
>
>> Hi Michael and others,
>>
>> maybe it's this one:
>>
>>
http://bjp.rcpsych.org/content/196/5/414.1.long
>>
>> best,
>> -joost
>>
>>
>> On Fri, Mar 23, 2012 at 2:15 AM,
Michael Harms
>> <
mharms@conte.wustl.edu>wrote:
>>
>>> Hi Jeff,
>>> I personally like the idea of
using average thickness as a covariate to
>>> control for a reduction in
"whole brain" thickness, and have used
that
>>> approach in a paper. If the
Abstract that you mentioned indicated that
>>> this is flawed, I'd be
curious to know what the reason was...
>>>
>>> cheers,
>>> -MH
>>>
>>> On Thu, 2012-03-22 at 21:00
-0400, Bruce Fischl wrote:
>>>> Hi Jeff
>>>>
>>>> yes, I think this is
still our recommendation for thickness,
although
>>>> perhaps David Salat can
verify. As far as surface area, you might
get
>>>> Anderson Winkler to send
you a preprint of his newly accepted paper
on
>>>> surface area comparisons
and how to do them properly. I would have
>>> said
>>>> normalize by the 2/3 root
of ICV (maybe David can comment on this as
>>> well)
>>>> cheers
>>>> Bruce
>>>>
>>>>
>>>> On Thu, 22 Mar 2012, Jeff
Sadino wrote:
>>>>
>>>>> Hello,
>>>>> For cortical
thickness normalizations, Bruce said not
to normalize
>>> based on a HBM
>>>>> abstract
>>>>> (
>>>
http://www.mail-archive.com/freesurfer@nmr.mgh.harvard.edu/msg06646.html).
>>> Is
>>>>> this still the
consensus?
>>>>>
>>>>> For cortical volume,
it is pretty standard to normalize to
eTIV.
>>>>>
>>>>> For cortical surface
area (jacobian), I couldn't find any
>>> information
>>> on the wiki.
>>>>> Does anyone have any
recommendations?
>>>>>
>>>>> Thank you,
>>>>> Jeff
>>>>>
>>>>>
>>>>
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