Specifying the channel layout for plotting
Introduction to layout files (*.lay files)
FieldTrip can use layout files that gives you exact control of the 2-D position of the sensors for topoplotting, and of the per-channel local coordinate axes for the multiplotting. These layout files are ascii file with the extention *.lay, or MATLAB files containing a variable with the name “lay”. In general, FieldTrip prefers .mat-files because of more flexible outlines of the headshape. For .lay files a standard headshape is drawn around the normalized channel positions. Here is a small snippet of an ascii layout file:
1 -0.308949 0.951110 0.750000 0.450000 Fp1 2 0.000121 1.000000 0.750000 0.450000 Fpz 3 0.309064 0.951004 0.750000 0.450000 Fp2 4 -0.808816 0.587705 0.750000 0.450000 F7 5 -0.411232 0.519845 0.750000 0.450000 F3 6 0.000257 0.499920 0.750000 0.450000 Fz 7 0.410919 0.519568 0.750000 0.450000 F4 8 0.809069 0.587789 0.750000 0.450000 F8 ...
The format of the layout file is described in more detail in the frequently asked question “what is the format of the layout file which is used for plotting”.
Standard layout files included in FieldTrip
The standard layout files included in FieldTrip are based on the sensor specifications as obtained from the manufacturer website or manual. If an image of the sensor positions is available on the manufacturer website, a copy of it is located in the fieldtrip/template/layout directory. For standard layout files that are included in FieldTrip, please check the corresponding getting started section corresponding of your system.
Using the ft_prepare_layout function to make a layout
The ft_prepare_layout function can be used to make a custom layout. This can be especially convenient if you have a nice bitmap image for the channel arrangement, like this
for the Easycap M10 electrodecap arrangement (see http://www.easycap.de/easycap/e/electrodes/13_M10.htm) for full details), or like this
for an ECoG electrode arrangement (this example photo is taken from Dalal et al. in Journal of Neuroscience Methods 174 (2008) 106–115).
You can specify cfg.image in ft_prepare_layout and subsequently click on the location of each electrode. After specifying each electrode location, you'll be asked to specify the outlines of the head (i.e. the circle around the head, the nose and ears and optionally some lines representing other important landmarks) and to specify the mask for the topographic interpolation.
cfg = ; cfg.image = 'dalal_ecog.png'; lay = ft_prepare_layout(cfg);
After creating the layout, you should manually assign the correct name of the channel labels in the lay.label cell-array. Furthermore, you probably should place the SCALE and COMNT locations at a convenient place in the figure and modify the width and height of the boxes used for multiplotting. You can use ft_layoutplot for a visual inspection of the complete layout
cfg = ; cfg.layout = lay; % this is the layout structure that you created with ft_prepare_layout ft_layoutplot(cfg);
or including the original image as black-and-white background like this
cfg = ; cfg.image = 'dalal_ecog.png'; % use the photo as background cfg.layout = lay; % this is the layout structure that you created with ft_prepare_layout ft_layoutplot(cfg);
Once you are happy with the result, you can save it to a MATLAB file:
save dalal_ecog.mat lay % save the layout in the variable "lay" to a MATLAB file
The MATLAB file can subsequently be specified as cfg.layout='dalal_ecog.mat' whenever you need a layout for plotting. Alternatively, you can specify the layout like cfg.layout=lay.
The advantage of the MATLAB file over the ascii file, is that the MATLAB file can also contain a user-specified outline of the head and user-specified mask, whereas for the ascii layout file always the same circle with nose and ears will be used.
Functions that require a layout configuration
Layouts are an essential element in FieldTrip, and many FieldTrip functions rely that the user specifies a layout in his configuration (cfg.layout). See below for an overview of such functions: