statistic on planar gradients

[Michael Wibral]

Dear Eric,

you suggested to,...

Use the following test statistic at the level of the (_dV,_dH) channel pairs: calculate the difference between the experimental conditions of the lengths of the trial-averaged vector-valued (_dV,_dH)-signals (J-M calls this “to Pythagoras”). You do this for all channels and all time points....

I disagree here, but not for mathematical/stastical reason, rather because of neurophysiology and the problem discussed in the thread before :
I think of two conditions that differ not in their amplitudes but have completely opposing direction of a dipole (in an identical location), then statitsics on raw fields will easily find this effect, but the length of the trial averaged vector valued signals will be identical (the dipole does not jitter across trials in this example, hence trial averaging of vector valued signals doesn't help. The dipole just inverts it's orientation across conditions). That's what I meant by multivariate statistics. Some importnat information gets lost (and must get lost) when going to a non-directional and unsigned quantity to test.

But maybe I simply misunderstood your suggestion altogether??

Michael

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