I.2: 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 of data components, an EEGLAB dataset must contain information about the scalp locations of the recording electrodes.
KEY STEP 5: Load channel location information.
To load or edit channel location information contained in a dataset, select Edit > Channel locations.
If you imported a binary data file in the Neuroscan or Biosemi formats, channel labels will already be present in the dataset (in EEGLAB v4.31 and later versions). When you call the channel editing window, a dialog box (shown below) will appear, asking you if you want to use standard channel locations based on the imported electrode position labels (for example 'Fz') from a channel locations file using an extended International 10-20 System.
You may choose between several templates. If you intend to perform source localization, we strongly suggest that you select the second option "Use MNI coordinates for the BEM Dipfit model" (the first set of 'BESA' coordinates was designed for a spherical BESA head model, now obsolete). For the sake of this tutorial, we will press the Cancel button and load the channel location file associated with this data manually.
To load a channel locations file, press the Read Locations button and select the sample channel locations file eeglab_chan32.locs (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 the button Read locs 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. We recommend using the default settings. Electrodes plotted outside the head cartoon are electrodes located below the mid-head line (i.e., with a negative z (height) coordinate, 0 being the center of the head). They are plotted outside the head cartoon by convention. To plot scalp maps only inside the head cartoon, enter 0.5 in the Plot radius edit box. In this case, the two eye electrodes will not be displayed nor taken into account when computing interpolated 2-D scalp maps for display or (in some cases) further processing. These settings are used for all scalp topographies plotted in EEGLAB. 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 possible scalp channel locations, with parts of the head below the (0.5) head equator shown in a 'skirt' or 'halo' region outside the cartoon head boundary (more precise separate controls of which channel locations to plot are available from the command line: see the 'Help' message for the scalp 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. NOTE: Equating 'channel locations' with (single) electrode locations only makes sense when all channels use the same 'reference channel.' An EEG channel signal is always the difference' between voltages at two (or more) electrodes -- typically some electrode "referred to" a reference channel (or channel combination. Equating the signal 'channel location' to the location of one of the contributing electrodes is quite imprecise, as the channel must be equally sensitive to potentials flowing to either of its two (or more) contributing scalp electrodes.
The appendix importing channel locations contains additional information.