After comparing analyzing some single pixel values with a standard GLM
statistical package (SAS and JMP), I think I now understand the solution and
problems associated obtaining a correlation between a difference in a
covariate obtained across a baseline and active condition and a difference
in the scans associated with each condition.  I would like to confirm that
my understanding is correct.

Given the caveat that there is only one scan per condition for each subject
and using the Conditions and Covariate design in SPM99, no covariate
interactions, no covariate centering, then as Andrew Holmes (and others)
have stated, a t-contrast of 0 0 1 will map the positive correlations
between the change in the covariate with the change in the scans.  This same
answer is obtained whether the covariate is entered as raw data, or mean
centered data, or whether the baseline covariate value is constant across
all subject (e.g., baseline covariate = 0).  The t-value given by SPM can be
converted to a correlation coefficient by the standard r to t formula using
N-2 for degrees of freedom (not the d.f. given in the SPM table).

The problem arises if you wish to derive the condition effects from the same
analysis.  In this case, the form of the covariate will make a great
difference.  If you use the raw covariate values, then the size of the
condition effect will be overestimated (i.e., larger t-value, lower p-value)
for the reasons explained by Karl Friston in recent postings.  One
unfortunate result is that a conjunction analysis between the condition and
covariate effects will be overly-liberal.

If the covariate is mean-centered for each condition, then the condition
effects will be properly estimated.  Alternatively, one could perform 2
separate analyses, one just to examine the covariate effects, and a second
condition-only design without the covariate to get the condition effects.

Is this correct ?

sg

( Note new e-mail address:
[log in to unmask] ) 
====================================
Steven Grant, Ph.D.
Cognitive Neuroscience of Addiction Program
Clinical Neurobiology Unit
Div. Treatment Research & Development
National Institute on Drug Abuse
Room 4-4238
6001 Executive Blvd
Bethesda, MD 20892 
301 443-4877 (voice)  443-6814 (fax)




%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%