Dear Nissen,
>>> The arrangement of the experimental condition in scan series was:
>>> AAAABBBBC1C1C1C1AAAABBBBC2C2C2C2AAAABBBBC3C3C3C3AAAABBBBC4C4C4C4
>>> AAAABBBBC1C1C1C1AAAABBBBC2C2C2C2AAAABBBBC3C3C3C3AAAABBBBC4C4C4C4
>>> AAAABBBBC1C1C1C1AAAABBBBC2C2C2C2AAAABBBBC3C3C3C3AAAABBBBC4C4C4C4
> ==>144 scans in total.
>
> First, there were 144 scans arranged as described above. Why is it that
> the eight values is enough? Should there be 12 values for the loading
> factor assignment?
Yes, the vector you specify for the parameterisation of condition C should
have 12 elements as there are 12 instances of condition C. Could you clarify
what you mean when you say that 'eight values is enough'?
> Second, acording to the simulation of reaction time data, the slope of
> the function for "loading factor from c1, c2, c3 to c4" is about 20.0
> linearly, is it reasonable to specify the 12 values for the parameteric
> factor --"loading" as "0 20 40 60 0 20 40 60 0 20 40 60" ? If not, what
> should I do?
Yes, this sounds OK. The vector you enter will be mean corrected
automatically. If you are looking for a linear relationship between loading
factor and BOLD activity in condition C, then select a polynomial expansion
with order 1. This will result in a design matrix with two columns for
condition C; the constant (zero order) term (representing the intercept of
the regression line relating activity in condition C to loading factor) and
the first order term (slope of the line relating activity in C to loading
factor). Thus you can test for a positive relationship between C and loading
factor with a contrast like [0 0 0 1].
If you would like to test for higher order relationships between loading
factor and C, then select a higher order polynomial expansion.
Best wishes,
Geraint
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Dr. Geraint Rees
Wellcome Advanced Fellow, Lecturer,
Mailstop 139-74, Institute of Neurology,
California Institute of Technology, University College London,
Pasadena, 12 Queen Square,
CA 91125 London WC1N 3BG
voice (626) 395-2880
fax (626) 796-8876
web http://www.klab.caltech.edu/~geraint
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