[Eeglablist] odd ERP images

[Dorothy Bishop]

I am a new EEGlab user and have been using EEGlab to explore a fairly noisy dataset gathered with 11-12 yr old children.
The paradigm involves presenting auditory stimuli; there is a train of tones (with SOAs of 250-450 ms), then a delay of 800 ms, then a new train. We are looking at the response to the first stimulus in a train. Given the long SOA prior to the stimulus, we don't expect to see any carryover from previous stimulus, and in adults we see a fairly standard auditory ERP, with small P1, then N1 (around 100 ms), P2.
In children of this age one does not always get N1 and P2 ; sometimes instead an initial P1 which then descends to a N2 around 250 ms.

The ERPimages look odd to me, and give concerns that there may be some kind of problem with how the paradigm was administered. It would be helpful to know if others have seen anything similar - if so, is there an explanation.
In short, I'd expect to be able to see vertical stripes in the ERPimage, given that this file just contains stimuli all of the same kind.
Instead, we see that pattern for a block of stimuli, but it then changes.  So what one sees are horizontal blocks in the image.
The ERPimage is not sorted, so the stimuli are just in the order in which they occurred. 
(50% of trials used a different tone frequency, and these have been removed, so the trials are not actually adjacent, in that there may have been intervening trials of a different kind. However, the same is seen regardless of which stimulus is used).
So it looks like the same kind of response occurred for 10-20 stimuli, but then it changed for the next 10-20, and so on.

I've put some sample ERPimages up on 
http://psyweb.psy.ox.ac.uk/oscci/Miscellaneous.htm 
Each ERPimage is from a different subject.  The stripey horizontal pattern is pretty common.






Dorothy Bishop
Professor of Developmental Neuropsychology
Department of Experimental Psychology
University of Oxford
OX1 3UD
 http://psyweb.psy.ox.ac.uk/oscci/

tel: +44 (0)1865 271369
fax: +44 (0)1865 281255