Introduction

When you measure skin conductance (also known as electrodermal activity (EDA) or galvinic skin response (GSR)), you are measuring how ‘well’ the skin conducts electricity. Often the skin will conduct electricity better during states of high arousal. Skin conductance is measured in micro Siemens (µS). Skin conductance levels usually vary between 2 and 20 µS.

Availability, support and advice

For availability of skin conductance equipment within your group, contact your lab coordinator. Skin conductance devices from Biopac and VU-AMS can be borrowed from SOLO through the Helpdesk. 

Protocol

Below, a general protocol of measuring skin conductance that is common at the FSW is described. Note that an adequate training is required before working with physiological data acquisition equipment and collecting such data from participants. If you are uncertain about the procedure, please contact your supervisor or lab coordinator. 

Materials

Since summer of 2023 Biopac EDA electrodes are not pre-gelled anymore. You have to apply the gel yourself.

Pre-gelled electrodes are labeled as EL507 and this newer dry electrodes as EL507A. The required gel is labeled GEL101A. With 1 bottle GEL101A you can gel approximately 200 electrodes.

How to apply the gel properly to the electrode:

• Shake the bottle with the nozzle down to settle the gel.
• Apply a small amount of the gel at a time and rub the gel into the sponge with the tip of the bottle.
• Be careful not to spread gel to the adhesive part of the electrode.
• Repeat the process until you have saturated the electrode and add just a little extra at the end. If you add too much, the gel will spread to the adhesive when you apply to the skin.

The above procedure is shown in this Biopac video

Preparation

Do not clean the skin beforehand using alcohol, abrasive substances (such as scrub gel), or soap. If the participant needs to wash his/her hands, just use lukewarm water without soap. The disposable electrodes come without gel. See the above procedure to apply gel. Always store the electrodes in a bag that can be sealed properly to prevent them from drying out.

Electrode placement

The placing of the electrodes can depend on the type of research. Before the start of the research project you should consult the literature and discuss the best approach with your supervisor. The best signal is measured on smooth, hairless skin. Active electrodes are placed on the inside of the fingers of the non-dominant hand (see figure). The electrodes can be attached to the distal phalanges (fingertips), as there is a greater responsiveness and the greatest sweat gland activity in that area. Another option is to attach the electrodes to the middle phalanges, as the fingertips are often callused. Place the electrodes on the index finger and middle finger, or on the index finger and ring finger. After you have attached the disposable electrodes you can attach the leads to the electrodes. It does not matter which leads you attach to which fingers. When you use the Biopac Bionomadix module, you can attach the strap with the transmitter to the participant’s wrist before attaching the leads to the electrodes. Ensure that the strap is placed between the Biopac transmitter and the skin, so the transmitter does not make contact with the skin.

When neither hands can be used (for instance when both hands are needed for the experiment, or with babies who are likely to pull on the electrodes), the inside of the foot can be used. In this case, the electrodes are placed on the musculus abductor hallucis (big toe abductor), bordering on the sole of the foot and halfway between the proximal phalanx of the big toe and the spot directly below the ankle (see figure, electrodes A and B). For more information about possible sites for measuring electrodermal response, see van Dooren, de Vries, and Janssen (2012).

Calibration

In the case of skin conductance it is necessary to calibrate before carrying out measurements. How this is done will differ according to the type of equipment. Always check how this should be done.

Afterwards

When you are finished, remove the leads and, if applicable, the wireless modules. The participant can then remove the disposable electrodes him or herself, and wash their hands. The disposable electrodes can be disposed of in the bin. After each participant, you should clean any equipment that has been in contact with the participant. This may apply to the leads that were attached to the electrodes and, if using the wireless Biopac module, the transmitter as well. Clean these components carefully with an alcohol wipe. The strap must be cleaned after use with Incidin Plus (see Incidin Plus protocol).

Tips & Tricks

• Check that the electrodes are gelled properly according to the above procedure.
• The participant should breathe slowly and regularly.
• The participant must move as little as possible to prevent artefacts in the data.
• The participant must be comfortable and sit in a natural posture with both feet on the floor.
• Before you start, check the signal. You can ask the participant to hold his/her breath briefly. Skin conductance should be going up while the participants holds his/her breath. That way you can check whether you are getting a good signal and whether the electrodes are attached properly.
• The figure below shows a typical skin conductance signal (recorded with BIOPAC AcqKnowledge over a period of about 6 minutes).
• When the data looks irregular or shows a flat line, check whether the leads are properly attached and whether the electrodes are still properly attached.
• If the quality of the data remains poor, it could be that the participant is a non-responder. This occurs in about 10% of all people.

Resources

• Boucsein, W., Fowles, D. C., Grimnes, S., Ben-Shakhar, G., Roth, W. T., Dawson, M. E., & Filion, D. L. (2012). Publication recommendations for electrodermal measurements. Psychophysiology, 49, 1017- 1034.
• Van Dooren, M., de Vries, J. J. G., & Janssen, J. H. (2012). Emotional sweating across the body: Comparing 16 different skin conductance measurement locations. Physiology & Behavior, 106, 298- 304.
• Biopac EDA faq