[VoxBo] Orthogonal contrasts including conditions labeled as zero

[Lila Chrysikou]

Thanks, Charan,
That's very helpful.
Best,
~Lila


On Jan 24, 2008 5:16 PM, Charan Ranganath <cranganath at ucdavis.edu> 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
>
>


-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Evangelia G. Chrysikou, Ph.D.
Post Doctoral Research Fellow
Thompson-Schill Lab

Center for Cognitive Neuroscience
Department of Psychology, University of Pennsylvania
3720 Walnut Street, Solomon Lab Room B51,
Philadelphia, PA 19104-6241

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
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