Dear SPM-users,
here comes an admittedly very basic question on a VBM analysis (SPM2, two
groups, GM and WM maps, three covariates. We are mainly interested in volume
differences and use modulated GM (WM) after optimized preprocessing).
Discussions came up, which model to use and how to encode the group variable.
(1) Multiple regression with constant: should I estimate two betas, i. e. use
one regressor 1 (..) 0 (...) and the other inversely? Is there an advantage
(apart from one degree of freedom) if effect coding is used, i. e. -1 and 1
in one regressor and estimate one coefficient?
or alternatively
(2) ANCOVA?
What role do different variances play in the two models?
Two more questions relate to the estmation of total GM and WM: can we simply
use the total signal in a modulated (cleaned) GM (WM) map?
For total intracranial volume (TIV): is the sum of the totals of the three
(GM, WM, CSF) (cleaned) modulated maps a sufficiently precise value to
control for interindividual head size / TIV?
Thanks a lot!!
Philipp
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