Author

Topic: Supraadditive effect

DianaR Junior Member

posted 15 May 2006 15:44
hi, I'm having trouble figuring out how to calculate a supraadditive 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 supraadditive, more than the sum of the two parts. How do I do this analysis (I have 10 subjects, 3 runs per subject)?

Fabri Moderator

posted 16 May 2006 14:40
I see no special problem in running the desired GLM contrast. You just have to specify in the Overlay GLM dialog, by checking condition A with + and conditions B and C with . This should be exactly what you are looking for, namely comparing the condition A to the sum of condition B and C effects.Many regards, Fabrizio quote: Originally posted by DianaR: hi, I'm having trouble figuring out how to calculate a supraadditive 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 supraadditive, more than the sum of the two parts. How do I do this analysis (I have 10 subjects, 3 runs per subject)?

DianaR Junior Member

posted 16 May 2006 15:23
Hi Fabrizio, 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 supraadditive 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. Am I interpreting this correctly? If not, can you please direct me in conducting this analysis?

Rainer Goebel Administrator

posted 17 May 2006 10:59
We tried such an analysis using conjunction of contrasts as described in the paper "Van Atteveldt, N., Formisano, E., Goebel, R., Blomert, L. (2004). Integration of letters and speech sounds in the human brain. Neuron, 43, 271282. Maybe you want to look at that paper for details. The approach suggested by Fabri ("+AV" with "A V") is fine if used unbalanced as you correctly note. When balancing such a contrast, you get a comparison of AV with the mean of A and V. A conjunction of contrasts might be better to sort out irrelevant areas, i.e. "(AV > A) AND (AV > V) AND (A > 0) AND (V > 0)". While this does not test for superadditivity, it masks out the relevant voxels, which can then be tested for nonlinearity (sub/superadditivity).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. Note also that true superadditivity is difficult to find in fMRI because of saturation effects, both at the neuronal and the hemodynamic level.
