Dear Will, SPM,
I have an experiment with two sessions. Each session has four activation
blocks. There is one condition per session.
I have a design matrix that implicitly models the baseline. My matrix
therefore has four columns: first for condition 1, second for condition
2, third and fourth for global intensity differences.
When I try to observe the activated voxels for cond1, I just ask a t [1
0 0 0]' contrast.
I make the same question for condition 2 (t [0 1 0 0]'). But now, say I
want to see differences between cond1 and cond2. If I perform a t [-1 1
0 0]' test, I firstly thought I would observe the areas in which cond2
is greater than cond1, that is, voxels more activated by cond2 than by
cond1. However, as my baseline is implicitly defined, I think that this
is not the case. So when I perform a t [-1 1 0 0] contrast I get the
differences between the conditions regardless of the baseline (i.e.
regardless of whether the voxel activated). Therefore I (suppose I) am
getting voxels that may or may not be active and that they are mostly
different.
My questions finally come.
The first one is whether I am right about the problems that assuming an
implicit baseline can generate in my final contrast.
The second one is what approach you think better, in the case that I
should change the way I create the contrasts. I could:
- Redesign my matrix including the baseline (BL) and test: cond1>BL AND
cond2>BL AND cond2>cond1
or I could
- Ask for a conjunction: cond1 AND cond2 AND cond2>cond1.
Thank you in advance,
Juan J.
PS: please find attached a figure with the design matrix, cond1 and
cond2 seen as a conjunction and a plot with the estimated cond1 and cond2
--
________________________________________________________
Juan J. Lull Noguera - jualulno_at_upvnet.upv.es
- [log in to unmask]
http://www.ibime.upv.es
[MI - Medical Imaging Area]
IBIME::Biomedical Informatics Group
UPV - Politechnical University of Valencia - Spain
________________________________________________________
[IM - Área de Imagen]
IBIME::Informática Biomédica
UPV - Universidad Politécnica de Valencia
________________________________________________________
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