Hi,
>>- Can this logic also be applied to ROIs that are dependent of the dataset
>>tested ie when ROIs have been generated using main effects of a factorial
>>design rather than using a functional localiser ?
>>
>>Concerning this last question, I am considering Friston et al 2006's remark
>>(A critique of functional localiser) which states :
>>*"The key thing to appreciate is that a contrast testing for a particular
>>effect can be used as a localiser for the remaining [orthogonal] effects. In
>>this sense, any factorial fMRI study has as many functional localisers,
>>embedded within it, as there are orthogonal contrasts. A typical two-by-two
>>design has three orthogonal contrasts. The natural conclusion is that all
>>fMRI experiments are simply collections of functional localisers."*
>
> The discussion in that paper and the one it argues with is very interesting.
> Friston's point, IIRC, is that the orthogonality of the contrasts means that
> there is underlying statistical independence, so the worries in the paper he's
> arguing with don't apply. (I can't remember if the independence of contrasts
> means that the underlying variables are truly independent, or only perhaps
> nearly so---depends on which quadratic form is used to define orthogonality,
> if my memory is correct.)
This reminds me of the recent discussion by Nikolaus Kriegeskorte et al:
"Another way to ensure independence is to use inherently independent
statistics for selection and selective analysis. For example, we may
select channels with a large average response to stimuli A and B
(contrast A + B) and test for a difference between the conditions
(contrast A – B). The contrast vectors ([1, 1]T and [1, –1]T) are
orthogonal. Unfortunately, contrast-vector orthogonality, by itself,
is not sufficient to ensure independence"
@article{kriegeskorte2009circular,
title={{Circular analysis in systems neuroscience: the dangers of
double dipping}},
author={Kriegeskorte, N. and Simmons, W.K. and Bellgowan, PS and Baker, C.I.},
journal={Nature Neuroscience},
volume={12},
number={5},
pages={535--540},
year={2009},
publisher={Nature Publishing Group}
}
Best,
Matthew
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