Dear SPM:
Sorry if the question is wordy, but I'm not sure how else to phrase it.
Thanks for your patience in reading it.
I have a question about PPI and whether 2 formulations are equivalent or
not. My example is let's say we have 2 groups of subjects performing a task
(patients and normals) and for simplicity the task is a block design
(active and control)
Method 1: P = psychological variable in this case 1 and -1 corresponding to
conditions.
WITHIN SUBJECT
Do canonical contrast [1 -1] based on comparing conditions within each subject
Extract signal from a region of interest (1st eigenvariate) = W
Set up a new analysis with 3 covariates [ [W x P] [W] [P] ] where W and
P are main effects of no interest.
This analysis says within subject are there areas which correlate with W as
modulated by the psychological variable.
BETWEEN GROUPS
Do for all subjects and then do a 2 sample t-test to see if patients and
normals differ.
Does the overall analysis say: Do the normal and patient groups differ in
regions which correlate with activity in area W as modulated by task?
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Method 2:
WITHIN SUBJECT
Do canonical contrast [1 -1] based on comparing conditions within each subject
BETWEEN GROUPS: P represents diagnosis
Take parameter estimate images for all subjects and enter into a 1 sample
t-test.
Extract signal (now representing parameter estimates) from a particular
region = B
Set up a new analysis with 3 covariates [ [B x P] [B] [P] ] where B and
P are main effects of no interest.
Does this overall analysis say: Are there areas which correlate with area
B, representing the difference between tasks, as modulated by subject
diagnosis?
I think these two answers are different but any insight would help.
Thanks,
Darren
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Darren R. Gitelman, M.D.
Cognitive Neurology and Alzheimer¹s Disease Center
E-mail: [log in to unmask]
WWW: http://www.brain.northwestern.edus
Voice: (312) 908-9023
Fax: (312) 908-8789
Northwestern Univ., 320 E. Superior St., Searle 11-470, Chicago, IL 60611
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