Hi,
I'm fairly new to VBM and am trying to get my head around differences
in testing for grey matter concentration or volume; specifically with
respect to why one would find a difference in concentration but not
volume in the same sample (as I have read in several papers).
From what I understand, the modulation step corrects signal
intensities for the volumetric contractions/expansions that occur
during spatial normalization, allowing absolute volume to be calculated.
I had an initial thought that, because the modulated volume measure
is adjusted for the deformations that occur during normalization, a
failure to detect a grey matter volume difference between two groups
might arise if one group had more heterogeneous brain morphology than
the other. This heterogeneity would lead to higher variability in the
deformation fields required for normalization, and therefore, the
corrections applied during modulation. This would then increase the
variance of the resulting grey matter volume measures, affecting the
statistics.
I later thought that grey matter concentration should still be
affected by this heterogeneity, given that it is calculated after
spatial normalization and should therefore be affeted by variability
in deformation fields. However, I noticed on a previous posting made
by John that he states "With different levels of registration
accuracy, there is a continuum between testing for GM volume
differences from the Jacobian determinants through to testing GM
volume differences purely from the conventional VBM point of view".
I was seeking clarification on what this continuum reflects, and
whether my thoughts are on track or totally misguided.
Thanks for your help,
Alex
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