Measuring Skin

Conductance

When many people feel threatened emotionally, they often sweat. The measure Skin Conductance can be used to more accurately quantify this common emotional response, even before the actual sensation of sweating is felt. As a result, the SCL measure has been used as a window into a subject's emotional state for many years. Electro-dermal Activity or EDA is another recent name for this measure.

"SCL" is Skin Conductance Level, the reciprocal of Skin Resistance. SCL is usually measured in microMhos (also referred to as microSiemens in Europe). This measure essentially assesses the ability of the skin to pass a (very small) current, and the conduction properties of the skin have been found to vary significantly as the subject responds emotionally to various situations.

The Skin Conductance Level measured on a subject has been found to depend on the following factors:

• The density of sweat glands in the area chosen
• The degree of psycho-activity of the sweat glands in that region
• The size of skin area in contact with the electrodes

Sweat gland density and psycho-activity (sensitivity or responsiveness) have been found to be most extensive on the palms of the hands and the soles of the feet. Greatest response has been found on the volar (palm-side) surfaces of the fingers. A common location used is to sense the subject's SCL using two electrodes placed on adjacent fingers of one of the subject's hands.

SCL is best measured by impressing a harmlessly small, CONSTANT VOLTAGE (0.5 volts in UFI SCL equipment) across the target segment of the skin surface (typically between two adjacent fingers), through two electrodes placed on the skin, and then measuring the change in voltage across a very small resistor in series with that segment. Of course, a special purpose signal conditioner is usually required for this. A subject's SCL changes slowly, over seconds, even as the subject responds to some stimulus. As a result, a 5 Hz or 10 Hz. sample rate is usually adequate for SCL recording.

"SCR" is Skin Conductance Response, which is the amplified change in the SCL over time. Capacitive coupling is used to remove the long term 'baseline', which is the SCL. The remaining signal is then amplified substantially. This allows very small, short-term fluctuations in the subject's SCL to be evaluated. Note that the SCR is derived from the SCL. In addition to the electronic approach mentioned above, the SCR can also be derived from the SCL in software.

This discussion has emphasized the Psycho-physiological use of the subject's SCL. The focus of this research is to measure the short term changes in the subject's SCL values in response to some sort of psychological stimulus. As a result, the actual site chosen for this psycho-physiological SCL (hands or feet) is actually selected due to a documented heightened response in Skin Conductance variability due to psychological factors at these sites.

The SCL can also be used to evaluate Hot Flashes. Hot Flashes usually develop and recede over minutes; i.e. a much longer time-frame than psycho-physiological SCL is interested in. As a result, SCL recording for HF analysis usually uses a different electrode site (across the Sternum) that has been documented to have a very suppressed response to short-term psychological aspects. In addition, HF SCL can usually be recorded at a slower rate -- 1 Hz. And SCL recording for Hot Flashes usually involves specialized electrodes and conductive paste, since typical recording periods for HF studies are 24 hours or longer. If you are interested in Hot Flashes, you can find a more detailed discussion here.

UFI has extensive experience with Skin Conductance signal conditioning and recording. Our model 2701 BioDerm(R) is a stand-alone Skin Conductance meter that can supply subject Skin Conductance Level and electronically derived Skin Conductance Response signals to your existing data collection system. The 2701 incorporates our proprietary Constant Voltage SCL signal conditioner, which supplies both Skin Conductance Level or SCL (the DC output) and Skin Conductance Response or SCR (the AC output). The 2701i just adds subject isolation to the capabilities of our standard 2701.

And, if you don't already have your own data collection system, we can supply our 2701-SC! This device starts with our 2701, then adds a built-in 12 bit data collection system and an isolated USB port. PC software included with the system gets SCL/SCR data to your computer over USB. The 2701-SC offers the simplest solution that we know of, to get your subject's SCL into your computer!

Both the 3991 and 3992 Biolog ambulatory recorders have been used extensively for Skin Conductance data monitoring. Both devices incorporate the same proprietary SCL signal conditioner, along with built-in data sampling (with 12 bits resolution) and recording. These Biologs can record just Skin Conductance, or we can add SCL recording to a wide range of other common measures you may want to record at the same time (ECG, IBI, Respiration, etc.).

And by the way, all our SCL systems use our 1081-FG electrodes specifically designed for SCL data sensing. The front face of these electrodes is generously contoured, and a hook and loop strap conveniently anchors these electrodes to your subject's fingers. And these Ag-AgCl electrodes are re-usable, which makes them a cost effective solution for SCL sensing.

If you still have questions, or if you don't see what you need, drop us a line, and we will see how we can help you!