Template:Tutorial Spatial Filter

Spatial Filtering

In the initial session you had a chance to gain some familiarity with the typical topography of a P300 signal. Generally, and as was likely the case for your own results, the P300 response is uniform over a large number of electrodes which implies that the spatial response has most of its energy in the lower frequencies. Mu responses, on the other hand, have a broader spatial response that includes more energy in mid and higher frequencies. Thus, with Mu responses, we will typically apply a spatial high-pass filter prior to performing an analysis. One example of such a filter is the common average reference (CAR) filter. Because this filter removes energy from the lower frequencies, however, it is generally a bad choice for P300 signals. For the purpose of this tutorial, we will show you how to configure BCI2000 so that no spatial filtering (i.e., identity spatial filtering) is applied to the signal.

Creating an Identity Filter

In its default configuration, BCI2000 requires the user to specify a spatial filter matrix. While there are spatial filters that may be effective for improving a P300 signal, we typically will not want to use a CAR filter. Instead we will specify a spatial filter that will not affect the signal (i.e., an identity spatial filter). For 8, 16 and 64 channel recordings, you may load any one of the parameter fragments available at parms/fragments/spatial_filters/IdentityN.prm where $N$ is the number of electrodes (e.g., 8, 16, 64). Applying one of these fragments will automatically set the appropriate values for the TransmitChList and SpatialFilter parameters. To set the spatial filter labels, which may be useful for subject-specific configuration, please review the steps below. If you didn't use an 8, 16 or 64-electrode array, you will need to configure the TransmitChList and SpatialFilter parameters as follows:

• From the main window of BCI2000 (i.e., Operator), click Config.
• Go to the Source tab. In the TransmitChList parameter, enter a space-separated list of channel numbers beginning with 1:
  1 2 3 4 ... N
• Go to the Filtering tab. Click the Edit Matrix button associated with the SpatialFilter parameter.
  • Set both Number of columns and Number of rows to $N$, and click Set new matrix size.
  • In the diagonal entries of the matrix, enter the value 1.
  • In the non-diagonal entries, enter the value 0.
  • Set the matrix labels. This step is not mandatory. However, it makes subject-specific configuration easier to set up later on.
    • Right-click the matrix, and choose Edit labels from the context menu.
    • In the first column of each row (i.e., the row labels), enter the appropriate 10-20 electrode location corresponding to that channel.
  • Close the matrix editor window.
• Click Configure Save in the configuration window, check all parameter entries except TransmitChList and SpatialFilter and close the window.
• Click Save Parameters, and save your spatial filter configuration under parms/fragments/spatial_filters/Identity<YourMontage>.prm.
• Click Configure Save again, uncheck all parameter entries and close the window.