BioSemi.nl

Hi:

We are using Actiview for recording data and Vision analyzer for analysis. While recording in Actiview, GSR data is displayed in microvolts, though the unit on the Auxilary sensors window says its Ohms. Even in the Analyzer, GSR data is displayed in microvolts. Could you please tell me whether GSR data is actually measured in Ohms or microvolts? Also, if it is measured in Ohms, why is it not displayed in Ohms?

Thank you

The GSR data is measured in Ohm, and this is also the "physical dimension" listed in the BDF header for the GSR channel (13th header field, see http://www.biosemi.com/faq/file_format.htm).
The problem with Brainvision Analyzer is that the header information concerning physical dimension is ignored. The program assumes that all channels contain data in uV (in case of GSR, 1 Ohm is displayed as 1 uV). For example, data from force and temperature sensors are also displayed in uV, even though the BDF header contains the correct "Newton" or "Celsius" unit information.

Best regards, Coen (BioSemi)

Hi,

Is there a macro to convert the GSR measured in ohms in Actiview system to microohms or micromhos?

Thank you

From Ohm to uMho (uSiemens): take reciprocal (1/x), and multiply with one million (1,000,000).

Beware: the reciprocal of a CHANGE in resistance (Ohm) is NOT a valid CHANGE in conductance (Mho), because: (1/x1 - 1/x2) is not equal to 1/(x1-x2). So, take the reciprocals of the individual data points before calculating changes in conductance.

Best regards, Coen (BioSemi)

When GSR is measured from the subject, is it measured in microvolts or volts?

Thank you

A constant current via the GSR elctrodes causes a voltage across the electrodes proportional to the impedance between the GSR electrodes. This voltage is measured and presented in the unit Ohm

Best regards, Coen (BioSemi)

[quote="Coen"]From Ohm to uMho (uSiemens): take reciprocal (1/x), and multiply with one million (1,000,000).
[/quote]

Hi Coen,

Based on this, and based on literature that suggests the threshold minimum for a GSR response is >0.02 UMho, we are getting excessively high responses, on the order of 1000-10 000x higher than one would expect. We've been at this for a while and keep coming back to it, but we still feel we don't have a good enough handle on the units in the data stream from Actiview to Analyzer. Do you have any further thoughts or insight?

Best regards,

Dan Zajdel

The ActiveTwo system uses an AC excitation current of 512 Hz, see viewtopic.php?t=18 . The reason for using this relatively high frequency is that we want to remain well outside the EEG bandwidth (to prevent interference from the GSR excitation to be visible in the EEG signal). Many stand-alone GSR devices use a much lower frequency, or even a DC current.

The impedances measured at higher frequencies are much lower (the electrode+skin impedances behaves like a capacitors). Consequently, ActiveTwo measures higher conductance levels (and higher conductance responses), than systems using a lower frequency excitation current.

The interpretation of the BDF units by various post-processing software can be checked by making some test recordings with fixed resistors.

Best regards, Coen (BioSemi)