Topographic Mapping

ERPLAB provides a simple interface to EEGLAB’s  topoplot function, which is accessed by selecting ERPLAB > Plot 2D ERP Map.  This routine allows you to plot the data from a selected time period for a selected set of bins from the currently selected ERPset.  The GUI is shown below, along with an example of the output.

To plot a set of topographic maps, you start by selecting which bin or bins will be plotted.  The popup menu allows you to see the labels for each bin so that you can remember what information is stored in each bin.  You next select the time period to plot.  You can select the Instantaneous amplitude, which plots the voltage distribution at a single time point (which you specify in the Latency(ies) field).  You can instead select Mean amplitude or Area amplitude, in which case you provide two time points to describe the beginning and end of the time range (mean and area are the same except that mean divides by the duration of the time period).  You must also select a baseline correction period, just as in plotting ERP waveforms.  Finally, you select a scaling option.  If you select Max-Min, the routine automatically chooses a scale for each bin on the basis of the minimum and maximum voltages found in that bin (e.g., the chosen scale might go from -4 µV to +9 µV). If you select Abs Max, the routine finds the minimum and maximum values, and creates a symmetrical scale depending on which has a greater absolute value (e.g., the chosen scale might go from -9 µV to +9 µV).  For these two options, each bin is scaled separately.  A third option is Custom, which allows you to specify the minimum and maximum values (e.g., you might put -5 +5 in the text box).  In this case, the scale is the same for each bin.

The topographic plotting routine requires that the location of each electrode is specified (not just the name, but the 3-D coordinates).  EEGLAB provides a set of tools for setting the electrode locations in the current EEG structure, accessed via Edit > Channel Locations, and this can be done very easily by using a set of standard coordinates based on the electrode names.  Ordinarily, you will do this prior to averaging.  If, however, you select ERPLAB > Plot 2D ERP Map without first setting the channel locations, ERPLAB will allow you to use EEGLAB’s channel location GUI to set the channel locations within the current ERP structure (see the screenshot below).  We typically find that we can simply accept the default settings.

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