For example, in the BV sample with LVF, RVF, and BVF (bilateral VF), imagine that blocks of LVF, RVF, and BVF were collected with rest in between blocks. My question is whether there are any regions that show more activity to BVF than to the sum of LVF and RVF -- indicating that BVF is supra-additive, more than the sum of the two parts.

Many regards,
Fabrizio

quote:

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Originally posted by DianaR:
hi, I'm having trouble figuring out how to calculate a supra-additive effect, meaning that activation to condition A is greater in a specific region than the sum of conditions B and C.

For example, in the BV sample with LVF, RVF, and BVF (bilateral VF), imagine that blocks of LVF, RVF, and BVF were collected with rest in between blocks. My question is whether there are any regions that show more activity to BVF than to the sum of LVF and RVF -- indicating that BVF is supra-additive, more than the sum of the two parts.

How do I do this analysis (I have 10 subjects, 3 runs per subject)?

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I've been thinking about this more, and I can explain better with a specific example:

I know that if I do the following in overlay GLM:
+ AV
- A
- V
and balance, the weights are 2, -1, -1
If I do a random effects analysis (10 ss, 3 runs each), at t=4.8, p<.001
I find a cluster of activation in the right STS of 8791 voxels to the AV stimulus. Other than bilateral STS, there are no other large activations showing AV > average (A, V)

If I do not balance the equation, the weights are 1, -1, -1
I thought this might be the additive approach I am looking for, since it doesn't balance the AV against the 2 unimodal conditions. The right STS activation is smaller at the same t value (2859 voxels), which initially made me think I was correct in interpreting this as a supra-additive effect (the AV effect is greater than the sum of the 2 unimodal effects, suggesting integration of the 2 modalities), but in this analysis there are a number of other activations (thalamus, retrosplenial cortext, visual cortex) and that doesn't make sense to me. If this is truly additive, there should be fewer active regions compared to the previous analysis, not more. AV against the sum of A and V should produce less activation than AV against .5A and .5V.

If you use the single, unbalanced contrast approach, you get everything active with regard to a difference without ensuring that your "right" side ("A", "V") is also activated - it could well be that the right side effects ("A" and/or "B") are zero or negative.