This section is a step-by-step tutorial in how to plot the ERPs following target trials, standard trials and their difference at all electrode locations organized in a scalp topography. This allows you to visually compare temporal and scalp distribution of the ERP, which can give you a good idea of your data. If all goes well, you should obtain a plot like this:
See the EEGLAB wiki for more information (scroll down to “the EEGLAB tutorial outline”).
Make sure the Current Directory is set to your NBT folder.
Type 'eeglab'. The screen in the figure below will appear.
Importing your data: File > import data > from BCI2000 .dat file
Select BCI Events 'StimulusBegin and 'StimulusType'.
Give the data set a name.
Re-referencing your data to Common Average Reference:
Tools > Re-reference
Choose Common Average Reference, click OK.
Give the new data set the same or a different name.
Because EEGLAB tends to get slower the more data sets are loaded, especially with large data files, you could delete data sets you are not going to use by selecting Edit > Delete dataset(s), entering its data set number and clicking OK.
Extracting the epochs: Tools > extract epochs
click the '…' button next to 'Time-locking event type(s)' and select StimulusBegin.
Choose an appropriate start and end window for the epochs, such as '0 0.8'. These values need to be entered in seconds.
The word 'epochs' is automatically added behind the name of the data set from which you extracted the epochs. Click OK if you agree with the name for the new data set.
If you want to save the new data set to disk, in the next window “pop_newset()” you can check the box 'Save it as file', enter a file name and click OK. If not, click OK as well without checking the box.
Epoch baseline removal: to make sure that the P300s of different electrodes are compared to a similar baseline signal, we subtract the average signal of 0 to 100 ms from the whole epoch (0 to 800 ms). Enter the baseline latency range '0 100'.
Load the electrode locations: this is required if you want to see the ERPs of all electrodes in a scalp topographic fashion. To do this, go to:
Edit > Channel locations
Read Locations (bottom left) > locate the file 'GSN-HydroCel-129.loc' (in NBT/supportfunctions) and double-click it.
Click OK in the next window where 'autodetect' is selected in the list.
Create separate data sets for target and non-target epochs:
Edit > select epochs or events
latency: 0 for min and 1 for max
type: select StimulusType from the '…' button
Click OK and confirm
Give the new data set an appropriate name, like “target responses”
Create data set with only non-target epochs by removing all target epochs:
Select the data set with the extracted epochs
Do the same, but now also select 'invert epoch selection'. A name could be “non-target responses”
Click OK and confirm
To plot the ERP waveforms: Plot > Sum/compare ERPs
Datasets to average: the number of the target data set
Datasets to average and subtract: the number of the non-target data set