[Eeglablist] Calculating Alpha Asymmetry

[Arnaud Delorme]

Dear Kathryn and Andero,

the eeg_regepochs is the command line function you want to use to epoch the data into regular (possibly overlapping) segments. For example

EEG = eeg_regepochs(EEG, 'recurrence', 1, 'limits', [0 2]);

Then you may go back to the EEGLAB interface using the code

[ALLEEG EEG] = eeg_store(ALLEEG, EEG, CURRENTSET);
eeglab redraw

Note that this function can also be used to insert events without extracting epochs. We will try to make a menu for it.
Best regards,

Arno

On Jan 5, 2012, at 6:24 AM, Andero Uusberg wrote:

> 
> Dear Kathryn,
> 
> all of your pipeline can be done in EEGLAB, but a few steps require 
> using Matlab command line interface.
> 
> For the segmentation of data EEGLAB has a function called 
> pop_importevent which does what the name suggests and thus allows you to 
> import event-markers for the 2-second epochs you'll need. As I'm not 
> quite sure how the function would cope with overlapping segments, I've 
> created non-overlapping ones at this phase and asked the spectral 
> analysis function pop_spectopo to apply overlapping during subsequent 
> FFT calculations instead.
> 
> Here is a piece of code that adds 2-second event-markers to a loaded 
> EEGLAB dataset.
> dur = 2; % the required duration of the new events
> name = 'M'; % name of the new events that appears in EEGLAB scroll plot
> newEvent = {name 0 dur}; % initiate the new event list (see help 
> pop_importevent for details)
> % the following loop will populate the list with required information
> for k = 2:round((EEG.xmax-dur)/dur) % the loop runs until the end of the 
> loaded dataset
>     newEvent = cat(1, newEvent, {name newEvent{k-1,2}+dur dur}); % 
> specs of one event are added on each run of the loop
> end
> EEG = pop_importevent( EEG, 'event','externalEvent','fields',{'type' 
> 'latency' 'duration'},'timeunit',1,'optimalign','off'); % import the 
> created event list to the loaded dataset
> eeglab redraw % update the GUI to reflect the command line changes
> 
> After running this code you can use the GUI-based tool to actually 
> segment the data (Tools -> Extract epochs).
> 
> The FFT can be calculated by the pop_spectopo function (Plot -> Plot 
> channel spectra and maps). See help pop_spectopo to learn how to tell 
> the algorithm to use overlapping in FFT calculations as well as which 
> kind of windowing to use.
> 
> Finally, you'll need to turn to the command-line again to perform the 
> subtraction of right and left power estimates. One way to do this 
> involves adding output variables to the pop_spectopo or spectopo 
> function (again, see help) and then performing the subtraction  on this 
> output. With some struggling the subtracted data can then be plotted 
> using EEGLAB or Matlab tools (e.g. plot) and exported (e.g. xlswrite).
> 
> Regards,
> Andero Uusberg
> Tartu, Estonia
> 
> 
> On 19.12.2011 3:51, Campbell, Kathryn wrote:
>> Hi,
>> 
>> I know there have been lots of questions about this but I have not found
>> any that answer all of my questions. I am completely new to using MATLAB
>> and EEGLAB and am a bit overwhelmed.
>> 
>> The steps I have taken in the past to calculate alpha asymmetry were
>> 
>> 1)Segment continuous EEG data into 4 one minute blocks based on events
>> markers
>> 
>> 2)For each block create a series of 2.048 second epochs overlapping by 50%
>> 
>> 3)Reject epochs with ocular or other artefacts (either manually or
>> automatically)
>> 
>> 4)Calculate mean alpha power at each electrode site for each block using
>> FFT with Hanning window.
>> 
>> 5)Use these figures to calculate alpha asymmetry (1n (R/L)
>> 
>> Unfortunately the system I now use to record EEG ( a very old version of
>> neuroscan) doesn’t seem have all of these functions available.
>> 
>> I am able to complete the first step using EEG lab but can’t find a way
>> to create overlapping epochs to move on to the next steps.
>> 
>> Is anyone able to help me? Please bear in mind I’m not that familiar
>> with MATLAB.
>> 
>> Thanks
>> 
>> Kathryn
>> 
>> 
>> 
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> 
> 
> 
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