source coherence in one condition

[Robert Oostenveld]

On 26 Apr 2007, at 0:35, Andrew Smart wrote:

> I would like to try to compute whether there is significant
> coherence across
> subjects but within one condition and between two dipoles. This code
> generates graphs but I am unclear about what they mean:

Hi Andrew,

It is unclear to me what the comparison is in the statistical test
that you want to perform. There should be a hypothesis like
"coherence is larger in XXX than YYY", which rephrased would be H0:
the coherence is the same. Based on the H0, the probablility can be
computed and H0 can (perhaps) be rejected. You cannot say that
"coherence is significant" just like that.

If the hypotheis is that coherence between one dipole pair is larger
than between the other dipole pair, then I am affraid that you cannot
test that using a permutation approach. With a permutation or
randomizarion test you test the hypothesis that the data in two
observed conditions is exchangeable. Simple differences in power in
one of the 4 sources underlying the two dipole pairs would already be
enough to reject the null-hypothesis that the data is exchangeable,
but those power differences do not have to mean that there is a
coherence difference.

Robert

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