Problem with data from BESA

Michael Wibral wibral at MPIH-FRANKFURT.MPG.DE
Mon Nov 21 15:21:48 CET 2005

Hi Robert,

thanks to your help I have meanfile figured out and solved the problems
with the topographies, i.e. the maps look fine now as far as geometry is
concerned. I guess the error was that the BESA files contained " ' " at
the end of the electrode names as the interpolation to a commen 81
electrodes was done using digitzed individual coordinates). I removed
the the extra " ' " and - just to make sure nothing goes wrong - also
made an ordered layout file for my configuration. What remains puzzling
however is the inversion of amplitudes  (+ -> - ). The exported .mul
file from BESA and the BESA data seem to match, however the plotted
data  seem to be inverted. I want to check this using simpler data,
though, and then come back to this if I can confirm it.

I have however another question regarding the interpretation of
clusteranalysis results. Am I correct in saying that the family wise
error rate (alpha) tells me the risk in obtaining a false positive
statement of the type that I specify previously with alphatresh? For
example if I specify alphathresh of 0.1 (lets calls this trend for
abbreviation) in the first pass of the analysis (multiple testing)
before clustering then the clusterrandomization using alpha =0.05 tells
me that I run a risk of 5% of identifying wrongly at least one of these
'trend clusters'.
(Or else, if the above is incorrect what is the reason not to use a very
lenient criterion in the first pass to feed the clusterrandomization
with as many clusters as possible?)

Best rgards,

Robert Oostenveld schrieb:

> On 16-nov-2005, at 18:56, Michael Wibral wrote:
>>  Both files also have a different order from the 10-10 layout used
>> in Fieldtrip, but I guess layout files match electrodes per name,
>> don't they. I will try to figure out a workaround.
> Channel matching is indeed done on name and not on number/index. This
> applies for the channel names in the layout file, but also for the
> channel names in the electrode file. It means that the channel
> ordering in either layout-file or elec-structure can be different
> from the channel ordering in the data, since both the data and the
> elec contain labels that can be matched when needed (e.g when
> plotting or dipole fitting). The elec-structure can also contain more
> or less electrode positions+labels than the EEG itself, e.g. when you
> have measured bipolar ECG or EOG along (without position), or when
> you have additional fiducials or electrodes in your cap that were
> recorded with a polhemus but not recorded as EEG channel.
> Since the sfp file is very simple and can hardly be read incorrectly,
> I suspect that the error in the assignment in channel names occurs in
> reading the ERP file.
> Robert
> .

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