Chapter 02: Channel Locations
Importing channel location for the tutorial dataset
To plot EEG scalp maps in either 2-D or 3-D format, or to estimate source locations for data components, an EEGLAB dataset must contain information about the locations of the recording electrodes.
KEY STEP 5: Load the channel locations.
To load or edit channel location information contained in a dataset, select Edit > Channel locations.
If you imported a binary data file in Neuroscan or Biosemi formats, channel labels will be present in the dataset (as of EEGLAB v4.31). When you call the channel editing window, a dialog box will appear asking you if you want to use standard channel locations corresponding to the imported channel labels (for example. Fz) from an channel locations file using an extended International 10-20 System. Otherwise, you must load a channel locations file manually.
To load a channel locations file, press the Read Locations button and select the sample channel locations file eeglab_chan32.locs (this file is located in the sample_data sub-directory of the EEGLAB distribution).
In the next pop-up window, simply press OK. If you do not specify the file format, the pop_chanedit.m function will attempt to use the filename extension to assess its format. Press button Read help in the main channel graphic interface window to view the supported formats.
Now the loaded channel labels and polar coordinates are displayed in the pop_chanedit.m window. To plot scalp maps only inside the head cartoon, enter 0.5 at the Plot radius edit box. In this case, the two eye electrodes will not be displayed nor considered computing interpolated 2-D scalp maps. If you do not see enough of the recorded field, set this dialogue box to 1.0 to interpolate and show scalp maps including all channels, with parts of the head below the (0.5) head equator shown in a 'skirt' or 'halo' outside the cartoon head boundary. (More precise separate controls of which channel locations to interpolate and plot are available from the command line in the topographic map plotting function topoplot.m. In the window below, you may scroll through the channel field values 1-by-1 using the < and > buttons, or in steps of 10 using << and >>.
Exploratory Step: Viewing Channel Locations.
Reopen Edit > Channel locations if you closed it. To visualize the 2-D locations of the channels, press Plot 2-D above the Read Locations button. Else, at any time during an EEGLAB session you may refer to a plot showing the channel locations by selecting Plot > Channel location > By name. Either command pops up a window like that below. Note: In this plot, click on any channel label to see its channel number.
The appendix section on importing channel location contains additional information.