BioSemi.nl

[dkrehbiel]

Hello ActiVIEW/ActiveTwo users,

I have an ActiveTwo system with a 16 Hz GSR module. I am confused about the scaling of the resulting data. I am currently reading the BDF files with EEGLAB (MatLab-based), selecting the GSR channel and saving the data as an ASCII file. My problem is that the resulting values are in the range of 20 some thousand, i.e. a far larger number for values in microsiemens than I ought to have. Any suggestions? Should I be referencing the GSR channel to something else (I have mastoid electrodes that I use for EEG recording at the same time)? Do I need to use some other software to read the data? If so, do you have suggestions (preferably cheap ones)?

Thanks for any clues.

Dwight K.

[Coen]

The GSR data for the 16Hz version is stored in nanoSiemens with a scaling of LSB = 31.25 nS (1/32th uS). This information is found in the file header. So, a value of 20,000 represents 0.625 microSiemens (= 1600 Ohm)

Best regards, Coen (BioSemi)

[cmuehl]

Dear Coen,

I believe that you mean 0.625 milliSiemens instead of microSiemens.

You mention that the smallest unit that can be measured (as I underrstand this is the implication of the LSB) is 31.25 nanoSiemens (nS). In my understanding that makes 20000 units = 20000*31.25 nS = 625000 nS = 625 microSiemens (uS) (which are indeed 1600 Ohm).

Please correct me, if I made a mistake at one point, as I try to understand a strange phenomenon in my data:

I am recording GSR during computer games. The signal looks good in its (smooth) form. However, the skin conductivity responses that I see are between 5 to 1000 microSiemens (sometimes higher). In the literature I find the notion of a normal phasic SC response being smaller by a factor of 10 or 100, i.e. between 0.5 and 10. I am using EEGlab for import of the .bdf, but also checked with BDF reader and EDF reader, so I exclude a mistake in conversion of units.

Could differences in the recording equipment used in different labs lead to the higher scaling of skin conductivity responses I observe?

Best,
Christian

[Coen]

Thanks for your comment, "micro" instead of "milli" was indeed a typing mistake.

The absolute values of GSR impedance are depended on many factors such as skin prepreration, electrode material, and frequency of the excitation current (16 Hz in our case). Large variations between subjects and experimental setups are indeed seen. By the way, you can easily check the calibration of the GSR with some resistors of know value. (connect resistor between 2 GSR electrodes, connect CMS and DRL to one of the GSR electrodes)

Best regards, Coen (BioSemi)