j.schoffelen at PSY.GLA.AC.UK
Mon May 18 22:13:05 CEST 2009
There is no fieldtrip function in the release allowing for the
testing of significant coherence, if I understand correctly what you
want. This would be to infer whether a given estimate of coherence is
significantly different from zero. This means that you are not
testing two conditions against each other (which is what
stafun_indepsamplesZcoh is doing; on top of this: it does it for all
combinations of channels, and not just for all channels versus a
given external reference signal).
There are two ways in which you can assess the 'significance'. First
would be something like a 'shift predictor'. In short, one could
shuffle the order of the replications for the reference channel and
recompute coherence between the reference and the rest. Provided the
reference is not the same signal in each trial, doing this many times
would give you an estimate of the 'bias' in coherence. Any observed
coherence values in the upper 5% tail then would count as significant
(this by the way only works for an external reference channel, and
not when you use one of the MEG channels as a reference).
This shift predictor is perhaps something of an overkill, and you
could use a parametric test instead. One way to do this, is to 'Z-
transform' the coherence. I am used to using a particular formula
which is described in one of our papers (Schoffelen, Science 2005
(supplementary material)); the appropriate references can also be
I hope this helps,
On May 16, 2009, at 4:05 PM, Nina Kahlbrock wrote:
> Dear Fieldtrip users,
> I have a question concerning significant coherence. I saw that a
> similar question was asked before on the mailing list. As I am new
> to coherence, I did not quite understand the answer.
> So here is my question: I have computed coherence between different
> MEG sensors and a photodiode. Now, I would like to compute
> significance levels for these coherence values at different
> frequencies. Is there a function in fieldtrip that I can use to
> solve this task? I have found the function
> ‘statfun_indepsamplesZcoh’. However, I am not sure whether this is
> the right one to use.
> I would greatly appreciate your help!
> Thanks in advance.
> The aim of this list is to facilitate the discussion between users
> of the FieldTrip toolbox, to share experiences and to discuss new
> ideas for MEG and EEG analysis.
The aim of this list is to facilitate the discussion between users of the FieldTrip toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.
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