[VoxBo] Orthogonal contrasts including conditions labeled as zero

[Charan Ranganath]

Lila, I think this is the answer to your question. When you do a t-test 
in voxbo (or spm, etc.) you are testing whether the value of a linear 
combination of beta values is significantly different from zero. So, if 
you weight A, B, and C as +1, +1, +1, you are testing the null 
hypothesis that (A+B+C)=0. Now, because "0" is your baseline condition, 
the beta values (parameter estimates) can be be interpreted as a measure 
of a change from this baseline. For convenience, let's call the baseline 
as "X". Therefore in this example, you are really testing 
(A-X+B-X+C-X)=0 or (A+B+C)-3X=0. If you multiply all of the parameters 
by some constant (say .33), then mathematically, it makes no difference. 
e.g., if (A+B+C)-3X=0, then .33A+.33B+.33C=0.

I hope this helps. CR

Lila Chrysikou wrote:
> Hello,
> 
> I was wondering how VoxBo treats any condition coded as zero (0) in 
> one's condition function if that condition is entered in a contrast.
> 
> For example, if I have three conditions A, B, and C and the null coded 
> as 0 in the condition function and I want to compare (A+B+C) with the 
> null, should I enter in my contrast.txt file the contrast coefficients 
> as 1 1 1 and assume that VoxBo will code the null as -3 to set up an 
> orthogonal contrast?
> 
> Or does VoxBo code the null as -1, hence I should enter my contrast as 
> .33 .33 .33 ?
> 
> Many thanks in advance for your help-
> 
> ~Lila
> 
> -- 
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Evangelia G. Chrysikou, Ph.D.
> Post Doctoral Research Fellow
> Thompson-Schill Lab
> 
> Office address: 3810 Walnut St.,Room 307, Philadelphia, PA 19104
> 215. 573.6726  (phone)
> 215. 898.1982  (fax)
> E-mail: evangelg at psych.upenn.edu <mailto:evangelg at psych.upenn.edu>

-- 
Charan Ranganath, Ph.D.
Associate Professor
Center for Neuroscience and Dept. of Psychology
University of California at Davis
1544 Newton Ct.
Davis, CA 95616

phone: 530-757-8750
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