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Bug 1339 - wrong number of sensors from bti2grad

Reported 2012-02-23 07:26:00 +0100
Modified 2012-04-02 16:26:45 +0200
Product: FieldTrip
Component: fileio
Version: unspecified
Hardware: PC
Operating System: Linux
Importance: P3 normal
Assigned to: Jan-Mathijs Schoffelen
Depends on:
See also:

Yuval - 2012-02-23 07:26:54 +0100

when using ft_read_header the "gradiometers" for our 4D248 layout are 271 in the grad structure we get 271 labels with 276 points, orientations etc. this is because bti2grad sums the number of coils and for some gradiometers 2 coils are given per point. I attached a header from read_4d_hdr. at the moment I fixed it by adding numcoils(numcoils>0,1)=1; just before totalcoils = sum(numcoils);

Yuval - 2012-02-23 07:29:57 +0100

Created attachment 231 hdr from read_4d_hdr

Jan-Mathijs Schoffelen - 2012-02-23 09:45:19 +0100

Hi Yuval, This is actually intended. The labels refer to the sensors, the pnt and ori to the coils. Coils don't have a label. The grad.tra matrix links the labels with the pnt/ori. Each row (corresponding to a sensor) gives the weights with which the individual coils contribute to that sensor. For unbalanced magnetometers there's a one-to-one relation between coils and sensors, a gradiometer requires 2 coils to be properly described. With five gradiometer references (out of 23 reference sensors), there are 5*2 + 18*1 coils. Add this to the 248 magnetometer coils of the true sensors you'll get to 276 Some more information can be found on: Note that since some time FieldTrip uses a slightly different representation of the gradiometers, making the distinction between sensors and coils more clear. To get to this new representation you can do: grad = bti2grad(hdr); gradnew = ft_datatype_sens(grad); Let me know if things are not clear. JM