[FieldTrip] Recording Biosemi GSR data in GDF files

[Eric Pohlmeyer]

Fieldtrip users:

Hi, my basic question is: Does anyone know how to record Biosemi GSR (skin conductance) data in a GDF file?  This is a different operation than just specifying additional non-EEG channels, as you do for the 8 external inputs on the Biosemi amp.

Some more details:

Currently we have been doing real-time transmission of EEG and the 8 external Biosemi channels using the Fieldtrip buffer and biosemi2ft, and all the data has been recorded fine.  We recently wanted to add GSR data to this.  However, it appears that the GSR data is treated differently by Biosemi than its other analog channels, thus when we add channel 265 (the GSR channel) to the biosemi2ft config file, we get another channel of data, but it is just the 16Hz excitation wave that Biosemi uses to deduce the GSR data, rather than the GSR data itself.  When we record the GSR data using Activew in a BDF file, the data is recorded fine.

Apparently Biosemi does some extra manipulations to GSR data in Actiview prior its storage in the BDF files, this extra manipulation occurs after it arrives from the USB driver, and is likely why we just see the excitation sine wave in the GDF file.  This is in contrasts to the 8 external channels which do not undergo such additional manipulations. 

So, before I dove in and tried to work out a solution, I was just wondering if anyone had already come up with a workaround for saving the Biosemi GSR data into the GDF?

Thanks!

Eric