[FieldTrip] EEG electrode coordinates

[Schoffelen, J.M. (Jan Mathijs)]

Hi Holly,

Although the coordinates might be ‘arbitrary’, they are defined with respect to a certain reference frame, right? If what you call the ‘fiducial’ locations are well-defined landmarks that are used to create one of the supported coordinate systems, then it would be relatively straightforward to map the points in your representation to coordinates that can be used for coregistration.

Procedure:

0) write some glue code, so that you can either read in, or represent your point cloud in a fieldtrip-like data structure (as would be outputted by ft_read_headshape). That is, either write a reading function (that can be plugged in), or convert your point cloud into a data structure (containing obj.pos at least) -> needed for step 2.
1) use ft_headcoordinates to create a transformation matrix that maps from your ‘arbitrary space’ to a known coordinate system. For this you need the fiducials to coincide with known landmarks.
2) use the derived transformation matrix to transform the coordinate system of your headshape object into the desired coordinate system (ft_transform_geometry).

Ideally, all steps are taken care of by ft_read_headshape, provided the file with the electrodes and fiducials allows for automatic extraction of the fiducial locations.

Best wishes,
Jan-Mathijs


On 23 Jul 2019, at 14:10, Holly Rossiter <RossiterH at cardiff.ac.uk<mailto:RossiterH at cardiff.ac.uk>> wrote:

Hi there,

Our student is using a novel method for coregistering EEG electrodes and fiducials using a number of cameras. We therefore have just a list of coordinates and fiducials that doesn’t come from a standard software package like polhemus for example. We are having trouble reading it in to use in ft_electroderealign/ft_volumerealign/ft_read_headshape in a form that fieldtrip is happy with as it asks for units and a coordinate system that it recognises whereas ours is sort of arbitrary. Is there any way to create the correct structure so that we can use these coordinates for EEG coregistration with an MRI?

Kind regards,
Holly
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