[Eeglablist] double event markers from Biosemi .bdf files

Arnaud Delorme arno at salk.edu
Tue Feb 15 14:02:04 PST 2005


Dear Jeff and Jim,

another possibility is that the events in your analog file are not 
perfectly encoded: there might be several values in the leading edge 
transition events. By default, EEGLAB will import one event per value. 
The solution to this problem is to import the BDF file without a channel 
event and without a reference (if you specify a reference and no channel 
event, the channel event will be re-referenced). Then go to menu item 
File > Import event info > From data channel. Enter your channel index, 
and select "3" instead of "1" for the leading edge transition maximal 
length. This will place the leading/trailing edge on the first/last data 
point where a transition is detected (it will assign the value for the 
type at the plateau). It might solve your problem. You may prevent the 
function from erasing the data channel (by clicking on the appropriate 
checkbox) and visualize the data channel along with the extracted events 
by typing on the command line

 >> eegplot(EEG.data(end,:), 'events', EEG.event);

Finally, do not forget to re-reference your data (otherwise there is a 
40dB loss in signal/noise).

Arno

ps: with some BIOSEMI files, you will also have to enter 
"bitand(uint32(X), 255)" in the "preprocessing transform" edit box of 
the GUI called by the menu item "File > Import event info > From data 
channel"

Jeff Johnson wrote:

> We frequently have this problem too, and we figure that it has 
> something to do with either pausing/unpausing or clicking the mouse in 
> Biosemi's acquisition program (in Labview). One way around it 
> (although we haven't tried) might be to pause/unpause the acquisition 
> by sending a byte (244 or 245?) from your stimulus computer, instead 
> of using the mouse.
>
> To get around it in EEGLAB, we usually just epoch our data around the 
> events that have a particular value. For example:
>
> pop_epoch( EEG, {'[1621761]'}, [-0.100 2.000] );
>
> Our events of interest always have a value of 1621761, whereas the 
> extraneous events have different values. You can easily find your 
> event values by using the pull-down menu in EEGLAB (edit >> event 
> values, I think). As you note, there are probably many other ways 
> around this problem, but this method works about 99% of the time for us.
>
> Jeff
>
> At 09:14 PM 2/14/2005 -0700, Jim Kroger wrote:
>
>> Hello, I am wondering if anybody else has experienced this problem. 
>> We use a Biosemi 128-channel.
>> Channel 129 has event markers.
>>
>> When using this channel to mark event times in EEGLAB, we have some 
>> kind of ring or artifact that causes EEGLAB to mark two events 
>> instead of one. We also try loading this into EMSE, and do not have 
>> the problem.
>>
>> We're having a bit of trouble understanding why this happens. 
>> Although we can imagine ways to program around the problem, it would 
>> be best if we understood it.
>>
>> Many thanks for any pointers.
>>
>> Jim
>>
>>
>>
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-- 

*Arnaud Delorme, Ph.D.*
Swartz Center for Computational Neuroscience, INC, University of San 
Diego California
La Jolla, CA92093-0961, USA

*Tel* :/(+1)-858-458-1927 ext 15/
*Fax* :/(+1)-858-458-1847/
*Web page*: sccn.ucsd.edu/~arno <http://www.sccn.ucsd.edu/%7Earno>
*To think upon*:

    Life is not a problem to be solved but a reality to be experienced.

        /Siren Kierkegaard/  

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