[Eeglablist] Epoching eeg data

Tue Sep 10 11:37:58 PDT 2013

Dear Katherine,

it does not make any difference to ICA whether the data are continuous or
epoched.
ICA looks for dependencies between channels forming independent component
within each sample - that is instantaneous dependencies are what creates
independent componenets. In the course of ICA the order of data samples
is even shuffled.

Instead of interpolating bad channels before ICA its far better not to
include
these channels in ICA (both the function and the GUI allow to pass indices
of electrodes to include in ICA).
You can interpolate these channels later (after removing artifactual ICs) if
your intent is to analyse sensor-level data.

I'm not sure about re-referencing to average before ICA. I've seen good de-
compositions of signals with such reference, but previously on the list
there were some comments that the decomposition can be better
with a different reference (Cz, mastoids, nose, whatever you want :) ).
You can perform ICA with average vs some other reference and compare
results.

Mikolaj

10 wrz 2013 19:43, "Katherine Naish" <K.R.Naish at pgr.reading.ac.uk>
napisał(a):

>  Dear all,
>
>
>
> The previous messages largely answer my query, but I'd like just to make
> sure I understand correctly. The results of ICA decomposition should be
> similar whether the data is epoched or not- is that correct? Obviously the
> results will be different if the data itself is different, but does ICA
> essentially treat epoched data as a continuous section?
>
>
>
> I would also really appreciate it if someone could approve, or otherwise,
> my pre-processing order. My pre-processing order is the following:
> resample, filter, epoch, remove baseline, interpolate bad channels and
> remove bad epochs (based on visual inspection), re-reference to average,
> and (finally) run ICA to detect artefacts. One thing I wasn't sure about
> was re-referencing- am i doing that at the right point?
>
>
>
> Many thanks in advance,
>
> katherine
>
>
>
>       ------------------------------
> *From:* eeglablist-bounces at sccn.ucsd.edu [eeglablist-bounces at sccn.ucsd.edu]
> on behalf of Mikołaj Magnuski [imponderabilion at gmail.com]
> *Sent:* 06 June 2013 19:13
> *Cc:* eeglablist at sccn.ucsd.edu
> *Subject:* Re: [Eeglablist] Epoching eeg data
>
>  Dear Karlo,
>
>  there is a case when dividing your whole 15 min. into smaller epochs
> (say 1 or 2 seconds long)
> is quite convenient with respect to ICA:
> --> cleaning data before ICA is easy if you just have to click bad epochs
> instead of marking
> bad periods in continuous data - although this is just my personal
> feeling.
> Nevertheless, you can also use automatic artifact rejection methods
> available in EEGlab this way. These methods require epoched data.
> (it is also easy to reconstruct your rejections later if you do them
> on epochs - you can just save the indices of windows that you removed [by
> coping relevant field in EEG structure after marking bad epochs,
> but before rejecting them]).
> --> Then, after your first IC decomposition, if you are not satisfied with
> its results and observe that
> some parts of your data may be responsible for poor results - you can
> remove
> epochs containing these data easily and perform ICA again.
>
>  So, just as Makoto wrote, ICA does not care whether data are epoched but
> you may -
> because in some cases it is convenient in preprocessing or
> 'postprocessing' (correcting
>  ICA decomposition).
> If you need any extra help with this - let me know (I have some scripts
> and functions to do this).
>
>  Mikolaj
>
>
> W dniu środa, 5 czerwca 2013 użytkownik Makoto Miyakoshi napisał:
>
>> Dear Karlo,
>>
>>  If you want to epoch your data into 3-min blocks, for example, put
>> event markers every 3 minutes then epoch the data.
>>
>>  ICA does not care temporal continuity of the data since it shuffles
>> them up differently for every iteration as a preprocess. That means, if the
>> total number of datapoints are the same before and after the epoching, the
>> ICA results should be the same.
>>
>>  Makoto
>>
>>
>> 2013/6/3 karlo gonzales <thats_karlo at yahoo.com<https://dbxprd0111.outlook.com/owa/UrlBlockedError.aspx>
>> >
>>
>>>  Dear EEGlab experts,
>>>
>>>  I need you help regarding an eeg data files which was recorded during
>>> eye closed state (15 min long).  Thus, there is no event to epoch the data.
>>> 1) How do you epoch such data?  2) should we epoch data before running
>>> ica (dose more epoch means better results?)
>>>  Thanks in advance
>>>
>>> _______________________________________________
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>>
>>
>>
>>  --
>> Makoto Miyakoshi
>> Swartz Center for Computational Neuroscience
>> Institute for Neural Computation, University of California San Diego
>>
>
>
> --
> Pozdrawiam,
> Mikołaj Magnuski
>
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