[VoxBo] Orthogonal contrasts including conditions labeled as zero

[Ranjani Prabhakaran]

Hi,

If I'm trying to do a region of interest analysis and want to compare 
all conditions (A,B, and C) versus baseline within a masked region, I 
found that if I set my weights for conditions A, B, C as +1, +1, +1 OR 
as +.33, +.33, +.33: I get the same t value from vbdumpstats.
But, if I want to get a beta value rather than a t-value, I found that 
I get different beta values depending on whether I set the weights as 
+1,+1,+1 or +.33,+.33,+.33.

What would be the appropriate weights for this contrast if I want to 
get a beta value?

Thanks!

Ranjani
On Jan 24, 2008, at 5:16 PM, Charan Ranganath wrote:

> 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
> fax: 530-757-8640
>
> http://DynamicMemoryLab.org