ADInstruments PowerLab Owner's Manual page 24

conductivity value is shown when everything is zeroed. Normal baseline conductivity
values should be in the range of 10 to 50 µS, depending on the individual and the air
humidity level (which aff ects skin conductivity). A scale measuring the change in
conductivity (also in µS) is used for the final record. This scale corresponds to the GSR
readout in a traditional polygraph.
Units
The GSR Amp measures conductivity directly in microsiemens (µS): units conversion is
done once the GSR Amp is zeroed.
Equipment and Technique
Measurement of GSR
Electrical resistance decreases (and conductance increases) between two points on the
skin when a subject experiences a stressful stimulus. This is known as the Féré eff ect,
aft er its discoverer. Terms such as psychogalvanic reflex and skin conductance response
have also been used to describe this eff ect. The modern term is galvanic skin response,
or GSR.
The change in conductivity on stimulation is due to an increase in the activity of sweat
glands, which provides a good general indicator of autonomic nervous system activity.
The increased conductivity arises through increased skin moisture, pre-secretory
activity of the sweat gland cell membranes, or both. The soles of the feet and palms
of the hands, including the palmar surfaces of the fingers and the plantar surfaces of
the toes, have the highest concentrations of these sweat glands (at least 2000 per cm 2 ,
about ten times higher than elsewhere), and so provide the best places to measure GSR.
The latest recommendations for the preparation of skin are described in Boucsein et al.,
2012. Skin preparation should consist of washing the hands without soap (it can cause
swelling of the epidermis), followed by rinsing and thorough drying. Alcohol should not
be used to clean the fingers as it penetrates and dries out the skin too much to allow
reliable measurement. Skin abrasion is neither necessary nor desirable (it reduces
sensitivity).
The electrodes supplied for use with the GSR Amp should be used with an electrode gel
or paste that contains a chloride salt such as NaCl or KCl. NaCl-containing electrode gel
is most commonly used, with concentrations in the range of 0.05–0.075 molar (0.3–0.4%
by weight) — this approximates the concentration of NaCl in the sweat that reaches the
epidermal surface.
The bipolar electrodes should be attached to the palmar surfaces of the fingers of one
hand with the attachment straps, firmly but not tightly (see Figure 8–6). Normally, the
middle segments (phalanges) of the first and second fingers are used, although you may
want to use the first and third fingers to remove any chance of contact between the
electrodes; consistent sites should be used in any event.
The subject should keep the hand still to avoid movement artifacts, and have a few
minutes to relax, to establish good baseline conduction — you will need to perform a
Owner's Guide
24