avg2csd converts the electric potential values in the input ERP average file avgfile to current source density values, and writes the results to the output file csdfile. csdfile has the same format as an ERP average file and can be used with other ERP programs as would an average file, but it must be kept in mind that the data contained in the file are no longer electric potential values in units of microvolts (uV), but current source density values in units of microamperes per square meter (uA/m2). The data type of csdfile is set to "csd", and the data can be plotted with ploterps(1) but almost certainly will need smoothing or filtering to be viewable. Smoothing or filtering can be done as the data are being displayed using the -smooth or -filter options to ploterps(1) , or a separate file containing filtered data can be created from csdfile using the filter(1) program. See the EXAMPLES section for typical examples of filtering.
The electrode locations about the scalp must be known in order to calculate the current source density values. Data collected with a standard electrode cap and standard channel labels have electrode locations known to avg2csd. However, if the data in avgfile were collected with a non-standard electrode cap, or the channel labels specified during data collection were something other than standard, it will be necessary to use the -es option (see below) with an electrode specification file.
avg2csd conaud.gnd conaud.csd ploterps xwin 26chan -files conaud.gnd -bins 7 ploterps xwin 26chan -files conaud.csd -bins 7 -filter lp 10
To do the same as above, by creating a filtered csd file:
avg2csd conaud.gnd conaud.csd filter lp conaud.csd conaud.csd.lp10 10 ploterps xwin 26chan -files conaud.gnd -bins 7 ploterps xwin 26chan -files conaud.csd.lp10 -bins 7
To create a csd file from an average file that has non-standard electrode placement (and/or non-standard channel labels), first create the ascii file concat03.esf (see topofiles(5) ), then run:
avg2csd concat03.avg concat03.csd -es concat03.esf