Authors: Francois Tadel, Martin Cousineau
The display of the time series figures can be configured using montages of sensors. The term montage in the Brainstorm interface can refer to a simple sub-selection of data channels, or a linear recombination of these channels (eg. average reference or other EEG re-referecing montage). The selection of channels was already introduced in the tutorials Continuous recordings and EEG and epilepsy. This page illustrates how to use the montage editor to create custom displays.
The interface offers some pre-defined montages for standard acquisition systems: MEG (Elekta-Neuromag, CTF, Yokogawa, 4D), EEG (10-20 and 10-10 caps) and NIRS (overlay of multiple wavelengths). The montage menu is accessible from the Record tab or with the popup menu on the time series figures.
The keyboard shortcuts (Shift+A, Shift+B, etc), allow to jump quickly to a different montage (see Assigning a keyboard shortcut). To display a second montage on the same recordings, open again the same file (right-click > Display time) and change the montage of the second window, as illustrated in the tutorial EEG and epilepsy.
To edit/create your own montages, use the menu "Edit montages". By default, it shows only the montages that are relevant to the type of recordings you are currently looking at (in the example below: EEG 10-20). To see all the available options, click on the button "All".
To create a new montage, click on the drop-down menu in the toolbar, the select the appropriate option (detailed in the following sections).
To create a new selection of data channels, select the menu New channel selection, enter a name for your montage, then select the channels you want to display in the list of available channels (hold the Shift or Ctrl/Command key to select multiple channels).
In the figure, the channels are displayed in the same order as they are defined in the channel file. If you want to change the display order of the channels, you need to create a custom montage.
All the other types of montages that you can edit are custom montages. They let you define line by line what to display in the time series figures. A montage is represented as a short text file, with one line of text per signal to display. A signal is a linear combination of one or more channels of data. The generic syntax to represent the sum of N channels chi with individual weights wi is:
label|color : w1*ch1, w2*ch2, ..., wN*chN
The screen capture below illustrates a few examples.
Line1: Channel F7.
Line2: Channel F8.
Line3: Subtraction (F7 - F8).
Line4: Subtraction (F7 - F8), displayed in green.
Color: Indicated in hexadecimal notation RRGGBB, use any HTML color picker for help.
Line5: Channel F7 in red and channel F8 in blue (overlay signals by giving them the same label).
Line6: Computes "FC1 - 0.5*F7 - 0.5*F8" (typical syntax for a linked re-referencing).
Separator: A line that contains only ":"
Line7: Channel ECG, with a fixed scaling factor 0.05 (so that it can be displayed in the same figure).
The menus New re-referencing montage (single ref) or (linked ref) are shortcuts to create custom montages that subtract the values of one or two channels to all the other channels.
Every time you select one of these menus, you get the warning below. The montages are not applied to the recordings, they are for visualization only. To re-reference correctly your EEG recordings, use the process "Standardize > Re-reference EEG".
A special type of montage is available for re-referencing the EEG/SEEG/ECoG recordings to an average reference. It subtracts from each channel the average of all the channels. It cannot be edited and is displayed as a matrix with (1-1/Nchan) on the diagonal and (-1/(Nchan-1)) everywhere else.
For ECoG and sEEG recordings, the average reference is computed separately for each group of contacts, typically each grid/strip or depth electrode. The groups of channels are defined with the field Group in the channel file, and can be edited with the channel editor (right-click on the channel file > Edit channel file).
Scalp current density
Another special type of montage is available for EEG only: Scalp Current Density (SCD). This commonly used technique applies a spatial Laplacian transformation to the EEG recordings, with the purpose of cleaning the signals or sharpening the topography maps. You can refer to the rich literature in this domain, and maybe start with this introduction video from Mike X Cohen:
In the example below, see how this montage can enhance the visual detection of spikes in the EEG /Epilepsy tutorial. Top=Average reference, Bottom=Scalp Current Density.
The implementation used here is the one developed by Robert Oostenveld in FieldTrip, the option 'finite' in ft_scalpcurrentdensity.m, which uses the function lapcal.m for the computation of the Laplacian.
Apply a montage to the recordings
The montage selection is only a visualization option, it never modifies the recordings. If you want to apply a specific montage to the recordings, you can use two processes:
Standardize > Re-reference EEG: Only available for continuous recordings, see EEG and Epilepsy.
Standardize > Apply montage: Only available for imported epochs, usually not recommended.
Assigning a keyboard shortcut
You can easily switch between montages directly from figures by assigning a keyboard shortcut to a montage (Shift+A to Shift+Z). Shift + A is reserved for All channels, but you are free to assign any other letter to the montage of your choice. In order to do so, select your montage of interest in the Montage editor and click on the keyboard icon.
You will then be prompted to enter a letter key (B to Z).
If a valid letter is entered, the shortcut will be automatically be saved and appear next to the chosen montage. If another montage had the same shortcut, it will be overwritten.