[Eeglablist] Oddly "Wavy" Data

[Matthew Stief]

Thanks Steve/Tarik/Andrew,

Of course it's alpha!  I knew it would be something stupid and elementary
like that.  My EEG education so far has been very narrow, basically I've
just been learning everything I need to know to troubleshoot this study as
issues arise.

I should have known given how incredibly mind numbingly boring my study is.

Thanks again.

-Matthew



On Mon, Feb 27, 2012 at 2:56 PM, Stephen Politzer-Ahles <
politzerahless at gmail.com> wrote:

> Hi Matthew,
>
> To me this looks a bit like alpha waves (common when a subject is getting
> tired, shutting their eyes, about to doze off, etc.) but I can't really be
> sure. I can't read the channel labels in your images, but alpha waves tend
> to show up the most robustly on posterior channels, so that would be one
> way to tell if that's what they are. If it is alpha waves, there are
> different opinions out there about what to do with them. Some experimenters
> usually reject stretches with alpha waves, whereas others believe they can
> often be left in if they're not too big (I believe Steve Luck's (2005) book
> has more information about this in the chapter on artifact rejection; I
> don't have a copy handy right now so I don't remember exactly what it
> says). In our lab we never try to remove them by filtering, because they
> tend to be in more or less the same frequency range as the
> (language-related) cognitive activity we're interested in; I'm not sure
> what frequency ranges you're interested in so I'm not sure if filtering
> would be appropriate or not. In my experience, alpha waves of about this
> size have seemed to get more or less averaged out of the ERPs if there are
> enough trials, but others on the list may be able to give more feedback.
>
> Good luck,
> Steve Politzer-Ahles
>
> On Mon, Feb 27, 2012 at 7:56 AM, Matthew Stief <ms2272 at cornell.edu> wrote:
>
>> Hi Everyone,
>>
>> I was wondering if anyone has had any experience with the "wavy"
>> activity that seems to go across almost all of the channels in this
>> subjects data.   I am sure it is familiar to you but I have not seen it in
>> any of the text books or guides I can find online and don't have an
>> experienced person supervising me.
>>
>> Here's what the strange activity looks like in filtered, cleaned,
>> epoched, and baselined data...
>>
>>
>> http://s1153.photobucket.com/albums/p512/mstief/?action=view&current=wavychannels7.jpg
>>
>>
>> Going back to the same dataset before any processing has been done, you
>> can see the same waves.  Obviously this is not particularly clean data, but
>> is it salvageable?
>>
>>
>> http://s1153.photobucket.com/albums/p512/mstief/?action=view&current=wavychannels8.jpg
>>
>>
>> Here's the exact same stretch of data after it has gone through 1hz high
>> pass and 55hz low pass filters.
>>
>>
>> http://s1153.photobucket.com/albums/p512/mstief/?action=view&current=wavychannels10.jpg
>>
>>
>> It does seem to be affecting the ERPs at least to my untutored eye,
>> here's one channel.
>>
>>
>> http://s1153.photobucket.com/albums/p512/mstief/?action=view&current=wavychannels4.jpg
>>
>>
>> So what is it?  Normal?  Horrible?  Fatal or merely deleterious?  Given
>> that it seems to contaminate most or all of the data for this person should
>> I just throw it out or can I get something out of it?  In particular how
>> will ICA handle it?  Do I still have a chance of getting a stable
>> decomposition out of it with a reliably identifiable P1?  This person was
>> particularly bad but there are less drastic examples in several of my other
>> participants.  Keep in mind some of my participants are rather rare and
>> would be difficult to replace (I study sexual orientation and some
>> varieties are less common than others).
>>
>> --
>> _________________________________________________________________
>> Matthew Stief
>> Human Development | Sex & Gender Lab | Cornell University
>> http://www.human.cornell.edu/HD/sexgender
>>
>>
>> Heterosexuality isn't normal, it's just common.
>> -Dorothy Parker
>>
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>
>
>
> --
> Stephen Politzer-Ahles
> University of Kansas
> Linguistics Department
> http://www.linguistics.ku.edu/
>



-- 
_________________________________________________________________
Matthew Stief
Human Development | Sex & Gender Lab | Cornell University
http://www.human.cornell.edu/HD/sexgender


Heterosexuality isn't normal, it's just common.
-Dorothy Parker
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