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  • Description
  • Computes the interpolated values of an evoked potential file (.p) at the original electrodes. Scalp potential or scalp current density (SCD, e.g., surface Laplacian) can be computed. The output is a new evoked potential file (.p).

  • Usage
  • epscd myfilein myfileout spline_order potscd_flag flag_regul [latbegin latend]

    with :

    • myfilein : input ep filename (potential data) (without extension).
    • myfileout : output filename (potential or SCD data) (without extension).
    • spline_order : order of the spline interpolation (usually 3 or 4).
    • potscd_flag : type of data to compute :
    • 0: data remains unchanged (potential values).
      1: computes SCD (Laplacian) from potential values.

    • flag_regul : type of regularization :
    • 0: no regularization.
      1: Tikhonov regularization.

    • options :
    • latbegin latend : begin and end latencies; optional parameters for computation. If omitted, computation is performed on the whole epoch.


  • Fields of parameter file and example

  • Examples
  • Command line to get the unregularized SCD for the whole epoch :
    epscd file_in file_out 4 1 0

  • Comments
    1. Spline order suggestion : if regularization is used for potential or SCD, take spline_order = 3, otherwise take spline_order = 4
    2. Possible error messages:
    3. Spline error : triangularization error (dspfa : 1) spline_calc_coef
      ERROR computing spline coefficients

      For these 2 messages, myfile_in probably contains electrodes for which SCD cannot be computed (aligned electrodes, too few electrodes, or incorrect electrode positions for example).

    4. Formulas :
    5. The comutation of SCD (Scalp Curent Density) is : ΔV*σ/R2 (en mA/m3) with
      σ = scalp conductivity = 0.45 Siemens/m = 0.45 (Ohm.m)−1
      R = head radius = 0.09 m
      σ/R2 = 55.55 103 (with V in μV)


  • Current version
  • 1.17 24-01-2011

  • History
    • 1.00 09-12-2001 (PEA) : 1st documented version.
    • 1.10 11-12-2001 (PEA) : minor modification.
    • 1.11 01-02-2002 (PEA) : minor modification.
    • 1.13 10-04-2002 (PEA) : minor modification.
    • 1.14 20-05-2002 (PEA) : minor modification.
    • 1.15 13-08-2007 (PEA) : minor modification.
    • 1.16 29-09-2010 (PEA) : updates to use cmake and free release of Elan.
    • 1.17 24-01-2011 (PEA) : removes static allocation for reading EP file.


  • Files
  • $ELANPATH/bin/epscd

  • See also
  • eegspline