[Eeglablist] Over-inflated degrees of freedom

Makoto Miyakoshi mmiyakoshi at ucsd.edu
Sun Nov 16 14:02:45 PST 2014

Dear Marco,

If I add my humble opinion to Steve's response, I would say that having 32
channels does not mean you have 32 degrees of freedom. They are highly
correlated, and the true resolution is much lower. SPM uses random field
theory to address this, and it uses resel (resolution element) instead of
voxcels. Usually, if there are 200,000 voxles, resel is around 5000. I have
never seen how it applies to EEG channels though.

So, using Fz, Cz, Pz... as independent measurement is basically wrong. If
you run ICA on 256ch, you'll usually find only 15-30 good ICs, which means
the true degrees of freedom should be close to that number. If you are
targetting e.g. P300, it is usually composed of 3-4 ICs only. You need to
run envtopo() to test this, which is available from GUI (but it has a weird
name on it).


On Fri, Oct 31, 2014 at 12:46 PM, Marco Montalto <montaltomarco at onvol.net>

> Dear List,
> I have conducted a study in which 20 participants (10 female and 10 male)
> were tested during 2 conditions using 32 electrodes placed on the scalp. I
> ran a mixed model ANOVA with within-subject factors condition (2 levels)
> and electrode (32 levels) and between-subject factor gender (2 levels). I
> got no significant main effects or interaction terms and therefore I
> collapsed across electrodes. Now each subject was represented by 32 values
> and therefore the degrees of freedom looked funny (they were huge) but the
> results were exactly as expected. If I took an average of each
> 32-value-cluster, again, I lost all significance. Does the method of having
> each subject represented by 32 values seem sensible to anyone and, if not,
> can anyone suggest a different method of analysis I could adopt?
> Thanks in advance for any advise/suggestions!
> Regards,
> Marco
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Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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