Galvanic Skin Response (GSRClosed) and Electrodermal Activity (EDAClosed) are the collective names for electrical phenomena of the skin. Most commonly measured are skin conductance level (SCLClosed) and skin conductance reaction (SCRClosed). Skin conductance is modulated by sympathetic activity and is not under conscious control. The GSR signal can therefore be used to gain an insight into the emotional state, or arousal, of the subject under specific experimental conditions.
Measurement principle
The Brain Products GSR module uses the exosomatic recording principle with direct current (DCClosed). This means that a constant voltage of 0.5 V is applied in order to acquire the skin conductance, according to published recommendations.
The GSR module converts the electric conductance of the skin to a voltage recorded by a bipolar amplifier input (AUXClosed Port). It is used together with surface electrodes that are in contact with the skin of the test subject.
Signal properties
The GSR signal is measured in µS (Siemens, physical unit of electric conductance). Physiological values are usually in the 1-30 µS range, however, significant inter-subject variability is to be expected within this range. The typical waveform is a composition of endogenous (1-5 spontaneous fluctuations/min) and evoked activity
A time-to-peak in the order of seconds is expected for evoked potentials. These timing characteristics make the GSR evoked potentials significantly different from classic event related potentials (ERPs) in the context of electroencephalography (EEGClosed). Therefore, during simultaneous GSR-EEG recordings, displaying GSR and EEG with the same time-scale does not allow one to assess significant variations in the signal. Consequently, our software for data recording, BrainVision Recorder, offers specific visualization options for peripheral signals as indicated in Recorder scientific view for GSR signal visualization.
Practical tips for the GSR measurement
The following tips allow optimal signal quality to be achieved with our GSRClosed sensor:The two electrodes should be placed on the index and middle fingers on the middle phalanx as shown below.
Only the GSR-specific gel designed to mimic the salt concentration of sweat ( Figure 1 ) should be used. EEGClosed gel should never be used.
Before starting the recording, the subject should have warm hands: cold or sweaty hands can have a confounding impact on the measurement.
The electrodes should not be taped too tightly to the finger (as not to affect the normal blood circulation).
Wait for approximately 5 minutes between the preparation of the electrodes and the start of the recording. The gel needs this time to create a good connection between the electrodes and the skin.
All Brain Products amplifiers with AUXClosed input support the GSR modules.
For the preliminary setup check prior to each measurement, or for pilot testing in general, a change in the signal can be elicited by performing the "Valsalva maneuver” consisting of a moderately forceful exhalation against closed mouth and nose.
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Autor:
Jackson Cionek
