[Eeglablist] [eeglablist] aliasing in pop_spectopo():

[Bradley Voytek]

Anders:

I don't really have time right now to look through the code (sorry)
but all you have to do is cast each channel of EEG data as a double,
remove the mean, and then recast it as a single. Then loop across
trials and replace EEG.data with this new mean-removed data.

Though I'm very curious to hear if Michael's suggestion fixed your issue.

::brad

On Wed, Oct 5, 2011 at 02:18, Anders Sand <engiltigadress at hotmail.com> wrote:
> Hi Brad and Alexandre,
> @ Alexandre - I tried applying a .5 Hz high-pass filter on the data. The
> spectopo do look nicer now (see attached). However, this solution do not
> seem to solve the problem at its root.
> @ Brad - I tried to do as you recommend. However, it does not fix the
> spectopo. Probably I pasted your loop at the wrong place, but I can't figure
> out where the misstake is. Do you mind looking at the attached m-file? If
> you search for "Brad" you will see where I pasted the loop.
> Thanks for you help!
> Anders
> ________________________________
> Date: Tue, 4 Oct 2011 17:13:11 -0400
> Subject: Re: [Eeglablist] [eeglablist] aliasing in pop_spectopo():
> From: alexandre.lehmann at gmail.com
> To: engiltigadress at hotmail.com
>
> Hi Anders,
>
> I guess these are your raw data ?
> What happens if you apply a hi-pass filter of 0.5 Hz ?
> I have seen some similarly weird looking spectra in someone's data from a
> neuroscan i guess and I think I remember that filtering helped out.
> Let me know if you find other ways around.
>
> Best
>
>> From: bradley.voytek at gmail.com
>> Date: Tue, 4 Oct 2011 13:15:06 -0700
>> Subject: Re: [Eeglablist] [eeglablist] aliasing in pop_spectopo():
>> To: engiltigadress at hotmail.com
>> CC: eeglablist at sccn.ucsd.edu
>>
>> Anders:
>>
>> This is due to overflow errors due to the BioSemi DC offset. You have
>> to remove the channel means *correctly* before running spectopo.
>>
>> See my same question and answer here:
>> http://sccn.ucsd.edu/pipermail/eeglablist/2008/002229.html
>>
>> And this conversation:
>> http://sccn.ucsd.edu/pipermail/eeglablist/2008/002148.html
>>
>> Good luck and let me know if you have any more questions!
>>
>> ::brad
>
>