Note that this reference documentation is identical to the help that is displayed in MATLAB when you type “help ft_datatype_sens”.

  FT_DATATYPE_SENS describes the FieldTrip structure that represents an EEG, ECoG, or
  MEG sensor array. This structure is commonly called "elec" for EEG, "grad" for MEG,
  "opto" for NIRS, or general "sens" for either one.
 
  For all sensor types a distinction should be made between the channel (i.e. the
  output of the transducer that is A/D converted) and the sensor, which may have some
  spatial extent. E.g. with EEG you can have a bipolar channel, where the position of
  the channel can be represented as in between the position of the two electrodes.
 
  The structure for MEG gradiometers and/or magnetometers contains
     sens.label    = Mx1 cell-array with channel labels
     sens.chanpos  = Mx3 matrix with channel positions
     sens.chanori  = Mx3 matrix with channel orientations, used for synthetic planar gradient computation
     sens.tra      = MxN matrix to combine coils into channels
     sens.coilpos  = Nx3 matrix with coil positions
     sens.coilori  = Nx3 matrix with coil orientations
     sens.balance  = structure containing info about the balancing, See FT_APPLY_MONTAGE
  and optionally
     sens.chanposold = Mx3 matrix with original channel positions (in case
                       sens.chanpos has been updated to contain NaNs, e.g.
                       after ft_componentanalysis)
     sens.chanoriold = Mx3 matrix with original channel orientations
     sens.labelold   = Mx1 cell-array with original channel labels
 
  The structure for EEG or ECoG channels contains
     sens.label    = Mx1 cell-array with channel labels
     sens.elecpos  = Nx3 matrix with electrode positions
     sens.chanpos  = Mx3 matrix with channel positions (often the same as electrode positions)
     sens.tra      = MxN matrix to combine electrodes into channels
  In case sens.tra is not present in the EEG sensor array, the channels
  are assumed to be average referenced.
 
  The structure for NIRS channels contains
    sens.optopos        = contains information about the position of the optodes
    sens.optotype       = contains information about the type of optode (receiver or transmitter)
    sens.chanpos        = contains information about the position of the channels (i.e. average of optopos)
    sens.tra            = NxC matrix, boolean, contains information about how receiver and transmitter form channels
    sens.wavelength     = 1xM vector of all wavelengths that were used
    sens.transmits      = NxM matrix, boolean, where N is the number of optodes and M the number of wavelengths per transmitter. Specifies what optode is transmitting at what wavelength (or nothing at all, which indicates that it is a receiver)
    sens.laserstrength  = 1xM vector of the strength of the emitted light of the lasers
 
  The following fields apply to MEG and EEG
     sens.chantype = Mx1 cell-array with the type of the channel, see FT_CHANTYPE
     sens.chanunit = Mx1 cell-array with the units of the channel signal, e.g. 'V', 'fT' or 'T/cm', see FT_CHANUNIT
 
  The following fields are optional
     sens.type     = string with the type of acquisition system, see FT_SENSTYPE
     sens.fid      = structure with fiducial information
 
  Historical fields:
     pnt, pos, ori, pnt1, pnt2
 
  Revision history:
  (2016/latest) The chantype and chanunit have become required fields.
   Original channel details are specified with the suffix "old" rather than "org".
   All numeric values are represented in double precision.
   It is possible to convert the amplitude and distance units (e.g. from T to fT and
   from m to mm) and it is possible to express planar and axial gradiometer channels
   either in units of amplitude or in units of amplitude/distance (i.e. proper
   gradient).
 
  (2011v2) The chantype and chanunit have been added for MEG.
 
  (2011v1) To facilitate determining the position of channels (e.g. for plotting)
   in case of balanced MEG or bipolar EEG, an explicit distinction has been made
   between chanpos+chanori and coilpos+coilori (for MEG) and chanpos and elecpos
   (for EEG). The pnt and ori fields are removed
 
  (2010) Added support for bipolar or otherwise more complex linear combinations
   of EEG electrodes using sens.tra, similar to MEG.
 
  (2009) Noice reduction has been added for MEG systems in the balance field.
 
  (2006) The optional fields sens.type and sens.unit were added.
 
  (2003) The initial version was defined, which looked like this for EEG
     sens.pnt     = Mx3 matrix with electrode positions
     sens.label   = Mx1 cell-array with channel labels
  and like this for MEG
     sens.pnt     = Nx3 matrix with coil positions
     sens.ori     = Nx3 matrix with coil orientations
     sens.tra     = MxN matrix to combine coils into channels
     sens.label   = Mx1 cell-array with channel labels
 
  See also FT_READ_SENS, FT_SENSTYPE, FT_CHANTYPE, FT_APPLY_MONTAGE, CTF2GRAD, FIF2GRAD,
  BTI2GRAD, YOKOGAWA2GRAD, ITAB2GRAD