[Eeglablist] search for locations of spike
arno at sccn.ucsd.edu
Fri Mar 5 09:01:22 PST 2004
>We are in search of the locations of epileptic
>discharges using 23 channels sampled at 400 Hz. We
>select epochs of 150 ms ( -100, + 50 ), 550 ms
>(-500,+50) and 1000 ms (-500,+500) centered on the
>peak of the found spikes ( around 80 ).
>We find out that the sources located by Dipfit are
>different for each epoch range used.
When you include more data, because you have few data channels (and as a
consequence few output components), ICA may focus on other EEG sources
>Question: what is the epoch lenght around the peak of
>the spike that should be used for the best fitting of
>components sources for epileptic spikes ?
The shorter the data epochs, the better. At the same time you must have
enough data points, so you need to find a trade-off. The best is to take
the decomposition where ICA components account for most of the epileptic
spikes (use menus Plot > Component ERP > With scalp map to estimate
>Should we sample our EEG at higher speed to get enough
>data points around the spike for reliable ICA
That is a solution, though in theory, sampling at a higher rate does not
bring much more information at low frequencies. The number of data
points with 150 ms epochs is 4800 which is much higher than 23^2 (529)
so it should be OK. If you want to localize the epileptic seizure
though, it would probably better to record data using many more channels?
To obtain a good decomposition, remember that you should carefully prune
your data of artifact and correct the baseline (or low-pass the
continuous data) as explained in the EEGLAB tutorial.
Hope this help
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* Arnaud Delorme <http://www.sccn.ucsd.edu/%7Earno>, Ph.D.* , SCCN,
UCSD, San Diego, USA
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