Skin Conductance

Skin conductance, SC (also known as Galvanic skin response, GSR, or electrodermal activity, EDA) is, as the name suggest, how conductive the skin is. This is used to calculate the amount of sweat a person is producing through their sweat glands as the sweat will increase the conductivity of the skin. Skin conductivity have been found to increase with arousal, one of the main dimension of an emotional response. The measurements have also been used to separate emotions such as anger and fear, and as data for the amount of stress a person is feeling. Skin conductance have also been used in wider areas such as psychopathology and personality disorders. The activation of sweat glands is controlled by the sympathetic division of the ANS.

There are two types of skin conductance which are tonic and phasic. Tonic skin conductance is the base conductivity when there is no particular stimuli (also called Skin conductance level, SCL). This value generally varies for different people. Phasic skin conductance (also referred to Skin Conductance Responses, SCR) is the conductivity when changes to the surrounding happens that causes the sympathetic division of the ANS to respond. SCR increases momentarily and will fall back to the base level. SCR can, however, occur without an identifiable stimuli (Non-specific SCR, NS-SCRs). Event-related SCR (ER-SCR) is SCR that are caused by specific stimuli. To measure the skin conductance, a balance has to be found between determining whether or not a SCR is event related. This will prevent NS-SCRs to be included in the analysis or not taking into account many ER-SCRs.

The graph above shows how the conductivity increases when there's a stimuli. However, when the subject got used to it, the stimuli no longer affect the user.

The table to the right summarises the different type of measure in Skin conductance.

Table 1 From A Guide for Analysing Electrodermal Activity (EDA) & Skin Conductance Responses (SCRs) for Psychological Experiments.

Skin conductance can be measured with silver electrodes attached to the fingers with a gel in between. The electrodes are usually placed at the lower part of the fingers close to the palm. The electrodes can also be used on the palm or the soles of the feet. These places have a high density of sweat glands allowing the measurements to be easier. Conductance is used is instead of resistance is due to how the sweat glands conduct electricity in a given area of skin. For parallel resistance, the total resistance will have to be calculate using:

(where the sweat glands acts as variable resistors) whereas conductance (measured in microSiemens) can be calculated simply by adding the conductance, CTotal = C1 + C2 + ... A small voltage of around 0.5V is applied across the electrodes and using the equation:

which is 1/Resistance to calculate conductance or the change in voltage.

This graph shows how the conductivity increases and rest time is needed before the next stimuli to accurately detect the start time.

Pg. 6 Diagram From A Guide for Analysing Electrodermal Activity (EDA) & Skin Conductance Responses (SCRs) for Psychological Experiments.

Challenges/Problems with Skin Conductance

There are limitations,however, when used in a natural environment to sense the user’s affective state. We use our hands frequently and different activities such as hand washing, eat or sports can cause the electrodes to move places and affect the signal quality.