[Eeglablist] search for locations of spike

[Arnaud Delorme]

>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 
and artifacts.

>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 
component contribution).

>Should we sample our EEG at higher speed to get enough
>data points around the spike for reliable ICA
>decomposition ?
>  
>
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
Best

Arno

>Thanks
>
>Camilo Borrego
>Universidad de Antioquia
>Medellin, Colombia
>
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-- 

* Arnaud Delorme <http://www.sccn.ucsd.edu/%7Earno>, Ph.D.* , SCCN, 
UCSD, San Diego, USA

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