GSR electrodes and signals

rwestafer · Post by **rwestafer** » Thu Jun 10, 2004 7:29 pm

I do not have any documentation regarding the nature of the GSR electrodes. How is the data acquisition performed (active, passive, etc), and what measure is recorded by ActiveTwo? Thanks!

Post by **Coen** » Fri Jun 11, 2004 11:55 am

We use a constant, alternating current via a pair of standard passive Ag-AgCl electrodes (1 microamp, 512 Hz). To determine the GSR response, the ActiveTwo measures the voltage drop between the two electrodes. Our ActiView acquisition software presents the GSR data in Ohm.

Best regards, Coen (BioSemi)

Guest · Post by **Guest** » Thu Dec 02, 2004 8:57 pm

Hi:

How is the average in GSR(Analyzer system) computed inside labview ?

Thank you

Post by **Coen** » Fri Dec 03, 2004 4:41 pm

Please refer to the ActiView diagram (source code). In subvi "Convert_4-7Switch.vi", the peak-to-peak value of the 512 Hz signal measured between the GSR electrodes is determined. The peak-peak values are resampled to the original 2048 Hz, and added to the data array as the GSR channel. In subvi "Sensor-M6-New.vi", the GSR channel is lowpass filtered with a cutoff frequency of 3 Hz (5th order Bessel filter), and displayed on screen. The GSR signal saved to file is unfiltered (bandwidth: DC-1/5th of the sample rate)

Best regards, Coen (BioSemi)

Guest · Post by **Guest** » Wed Jan 12, 2005 7:10 pm

Hi:

Please clarify these questions for me.

1) What is the measurement that comes through the GSR electrodes?
2) What measurement is recorded in Actiview and Analyzer?
3) If resistance is the measurement, then is it the resistance itself or the change in the resistance?
4) If resistance is measured, is negative resistance possible?
5)What does the display in the analyzer indicate?---the recorded measurement or the converted measurement?

Thank you

Post by **Coen** » Wed Jan 12, 2005 10:32 pm

1) and 2) Already answered above: the impedance between the GSR electrodes is measured and recorded in ActiView.
3) The absolute impedance is measured and saved to file, only the changes are displayed in ActiView.
4) Negative impedances cannot be measured.
5) The BDF file contains data converted to impedances (in Ohm).

Best regards, Coen (BioSemi)

hochmang · Post by **hochmang** » Thu Jan 10, 2008 9:11 pm

Hello,

I've been using EEGLAB to analyse the GSR data and got some negative values. What these value mean? I also get the results in microvolt. What kind of transformation should I do on the values to get the true impedance in Ohm?

Thank you very much

Post by **Coen** » Fri Jan 11, 2008 7:31 pm

For the (old) 512 Hz version, the unit is Ohm, and the value in the BDF file is scaled as LSB = 0.03125 Ohm
For the (new) 16 Hz version, the unit is nanoSiemens (0.001 uS), and the value in the BDF file is scaled as LSB = 0.03125 nanoSiemens

ActiView cannot generate negative values (either Ohm or nanoSiemens) for the GSR results. The maximum values are respectively 262 kOhm (2^23 * 0.03125 Ohm) and 262 uS (2^23 * 0.03125 nS).

Siemens (conductance, admittance) is the reciprocal of Ohm (resistance, impedance).

To determine whether your system has a 512 Hz or 16 Hz GSR circuit, check the unit displayed in ActiView during acquisition (Ohm for 512 Hz version, nanoSiemens for 16 Hz version), or check the 13th field in the header of the BDF file (http://www.biosemi.com/faq/file_format.htm). We offer a free upgrade to the 16 Hz version, see viewtopic.php?t=84&highlight=gsr

Best regards, Coen (BioSemi)

BioSemi.nl

GSR electrodes and signals

GSR electrodes and signals

GSR resistance measurement