[Eeglablist] Problem with epoching and boundary events

James Desjardins jdesjardins at brocku.ca
Mon Jan 13 11:08:21 PST 2014


Hi Phillipp,

I have posted your issue to EEGLAB Bugzilla (bug# 1552) and committed the fix to the EEGLAB SVN repository (revision 10261).

Let me know if my replication of your account captures the issue that you described (taken from the bugzilla report):

I replicated the problem by creating three events in a continuous dataset
containing a boundary discontinuity at 4.2 seconds into the recording. I added
event type 's3' (three seconds into the recording), event type 's4' (four
seconds into the recording) and event type 's5' (five seconds into the
recording).

If I epoch the data (+/- .5 seconds) to only 's4' I appropriately get "no epochs" related
errors.

If I epoch to {'s3' 's4'} I get 2 epochs and the boundary event is missing from
the data discontinuity in the second epoch.

If I epoch to {'s3' 's4' 's5'} I get 3 epochs and the boundary event is missing
from the data discontinuity in the second epoch.

If I add a boundary discontinuity at time 5.2 seconds into the recording and
then epoch to {'s3' 's4' 's5'} I get 2 epochs, one for 's3' and the second for
's5'. In this case again the boundary event is missing from the data
discontinuity in the second epoch (for 's5'), but it appropriately dropped the
's4' discontinuous epoch.

The issue of the final boundary event in the segmented dataset being dropped
can be traced back to line 505 in the current eeg_checkset function where it is
removing the final boundary event if its latency is greater than the size of
the second dimension of EEG.data. This step only seems to be appropriate when
the data are continuous (EEG.trials=1).

 
Thanks for noticing and reporting this!


James Desjardins, MA
Electrophysiology Technologist
Cognitive and Affective Neuroscience Lab, Psychology Department
Jack and Nora Walker Centre for Lifespan Development Research
Brock University
500 Glenridge Ave.
St. Catharines, ON, Canada L2S 3A1
905-688-5550 x4676
--
"'Cause you never can tell What goes on down below!
"This pool might be bigger Than you or I know!"

McElligot's Pool
Dr.Seuss 1947

________________________________________
From: eeglablist-bounces at sccn.ucsd.edu [eeglablist-bounces at sccn.ucsd.edu] on behalf of Philipp Spachtholz [Spachtholz at psy.lmu.de]
Sent: January-10-14 6:20 AM
To: eeglablist at sccn.ucsd.edu
Subject: [Eeglablist] Problem with epoching and boundary events

Dear all,

i am new to EEGLAB and started doing some scripting and came across a
problem. Maybe
someone can help me.

I import data from Brain Vision Analyzer (where i marked bad data
stretches). I then read out the
datapoints of these bad strechtes using the event "userdefined" and
subject those values to
eeg_eegrej. This seems to work well and the bad stretches are replaced
with a boundary event.

However when I do epoching over all stimulus events (each stimulus event
only occurs once in my experiment), some of them are epoched although
the interval contains a boundary event.
For example i have a Stimulus type 'S 79' which is followed by a
boundary event within 1 second. If i specifically select this event for
epoching with a time range of [-0.6 1.8] i get an error (which makes
sense to me).
If i select stimulus type 'S 79' and the succeding event after the
boundary event (or another one), only the event
after the boundary is selected for epoching --> I get 1 epoch.

However when i select more than 2 stimulus types for epoching, e.g. 3
events then I get 3 epochs,
including the S 79.

Do you have any idea why this might happen?
This happens both using the script and the gui.

thank you very much for your help

Philipp Spachtholz


--
Dipl. Psych. Philipp Spachtholz

Department of Psychology
University of Munich
Leopoldstrasse 13
80802 Germany
Phone: +49 (89) 2180-5229
E-Mail: Spachtholz at psy.lmu.de

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