EEGLAB Tutorial: Import Events

Get your data ready

EEGLAB can be used for the analysis and visualization of EEG datasets recorded using OpenBCI hardware and software. EEGLAB can work with a variety of different file types, including those that are exported from the OpenBCI GUI, as we saw in the previous post.

Events File

If you are working with Motor Imagery, Event Related Cortical Potentials (ERPs) or you just interested in the data followed by a specific moment, you need to upload Events Information in a “.txt” file.

Since I am working with Motor Imagery and Motor Execution and I am using OpenBCI axelerometer data to save my events, I wrote an script that finds all the events that I am interested in.

% Trials Events 
RF_Re = find(Trials == 1); % Move Right Finger 
LF_Re = find(Trials == 2); % Move Left Finger 
RF_Im = find(Trials == 3); % Imagine Moving Right Finger 
LF_Im = find(Trials == 4); % Imagine Moving Left Finger 
Events_row = [RF_Re; LF_Re; RF_Im; LF_Im]; 
% List of events (based on counts) 
E = sort(Events_row); % List of events (based on counts) sorted 

% Two External Buttons 
RightButton = find(Right == 25120); % RightButton is pressed 
Events_RB = RightButton*4-4; % Translate the row to time 
for RB_Char = {'RightButton'}; % Create 'Right Button' column 
RB = repmat (RB_Char, length(Events_RB),1); 
end 
R = table(Events_RB,RB); % Create a table with Right Button events 
LeftButton = find(Left == 25120); % LeftButton is pressed 
Events_LB = LeftButton*4-4; % Translate the row to time 
for LB_Char = {'LeftButton'}; % Create 'Left Button' column 
LB = repmat (LB_Char, length(Events_LB),1); 
end 
L = table(Events_LB,LB); % Create a table with Left Button events 

% OBSERVATION: Some events are not saved. There is a difference of 2506ms 
% between two different type of events (RF_Re and LF_Re, for example). 
% This means that between two values of the same event there is difference 
% of 10000ms. Currently, you must add this new values manually. 

MoveRF = RF_Re(1,:); % Find 1st Move RF Event 
MoveLF = LF_Re(1,:); % Find 1st Move LF Event 
ImagineRF = RF_Im(1,:); % Find 1st Imagine RF Event 
ImagineLF = LF_Im(1,:); % Find 1st Imagine RF Event 

Move_RF = (MoveRF:10000:(MoveRF+100000-1000))'; 
% Create Move RF Event Variable 
Move_LF = (MoveLF:10000:(MoveLF+100000-1000))'; 
% Create Move LF Event Variable 
Imagine_RF = (ImagineRF:10000:(ImagineRF+100000-1000))'; 
% Create Imagine RF Event Variable 
Imagine_LF = (ImagineLF:10000:(ImagineLF+100000-1000))'; 
% Create Imagine LF Event Variable 

% Conversion of variables (From counts to time) 
Events_RFRe = Move_RF*4-4; % Translate MoveRF counts into time 
Events_LFRe = Move_LF*4-4; % Translate MoveLF counts into time 
Events_RFIm = Imagine_RF*4-4; % Translate ImagineRF counts into time 
Events_LFIm = Imagine_LF*4-4; % Translate ImagineLF counts into time 
Events = [Events_RFRe; Events_LFRe; Events_RFIm; Events_LFIm]; 
% List of events (based on time) 
Events_Time = sort(Events); % Sort of the 2nd List of Events

Also, to make my life easier, I wrote a legit script to save the events accordingly to EEGLAB requirements. , where all the events are label and sort in time.

 % EEGLAB Event file (.xls)
 % Latency (1st Column)
    Latency = sort(Events); % Sort of 1st Events List (EEGLAB variable)
 % Type (2nd Column)
    for Cues = {'MoveRF';'MoveLF'; 'ImagineRF';'ImagineLF'};
      % Repeat matix 10 times (10 trials) in column 1
      Type = repmat(Cues, 10, 1);
    end
 % Duration (3rd Column)
    for Sec = 4;
      Duration = repmat(Sec,40,1);
    end
 % General Variable
    EEGLAB_Events = table(Latency, Type, Duration);
 % Save EEGLAB_Events.xlsx in Excel File (Ready to upload in EEGLAB)
    filename = 'EEGLAB_Events.xlsx';
    writetable(EEGLAB_Events,filename,'Sheet',1,'Range','A1');

 

Thus, the result is a “.csv” file distributed in three different columns:

• Latency column has the specific times event happend sorted by time.
• Type column distinguish among the different types of events.
• Duration column shows the length of the event.

Figure 1. Events File format

Plot your data into EEGLAB using the EEGLAB GUI: Plot -> Channel data (scroll). By default, EEGLAB has a 5 seconds window length (x axis). If you want to change it, you must select Settings -> time range to display in the figure window and type the new window length (s).

Figure 7. Plot EEG data and changing window length(s).

Besides, you can also change the scale of the plot by playing a little bit with the “+” and “-“in the bottom of the plot. I would suggest a scale of 50 or 100. Thus, you will have better perspective and visualization of EEG data. This should be the result:

If you wanna go one step further, you can import events into your data by using the EEGLAB GUI: File -> Import event info -> From Matlab array or ASCII file. Make sure to write in Input field “latency type duration”, and the following parameters showed in Figure 8.

Figure 9. Import Events File

At this point, you can plot EEG data with the events marked in different colours by following the same steps as earlier when we plot only EEG data.

Figure 10. EEG data with different events ploted.

There is also the opportunity to reject continous EEG data so as to “clean” your data by selecting with the cursor the length of the data that you want to remove from the signal. Although I will explain in another post how to do it significantly, feel free to play with it!

Figure 11. EEG data ploted with potential rejections.

References

• EEGLAB Tutorial: http://sccn.ucsd.edu/wiki/EEGLAB_TUTORIAL_OUTLINE
• EEGLAB Wiki: http://sccn.ucsd.edu/wiki/EEGLAB

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