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 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. 

Setup

Equipment

Skin conductance can be measured with the BIOPAC EDA100C/GSR100C (wired), PPGED-R (wireless), or EDA100D (wired smart amplifier) modules, or with VU-AMS equipment.

Leads and electrodes

EDA100C/GSR100C
The EDA100C and GSR100C are used with two LEAD-110 (black, unshielded) leads connected to VIN+ and VIN-. Usually, skin conductance is measured with dry disposable EDA electrodes on which some isotonic gel is applied (see Materials for instructions).

PPGED-R
With the PPGED-R you can measure both PPG and skin conductance (EDA). The PPGED-R transmitter can be attached to a BioNomadix velcro strap with size 33 cm (BN-STRAP-33), which can be tied around the wrist of the participant. Use the BN-EDA-LEAD2 to measure skin conductance. Usually, skin conductance is measured with dry disposable EDA electrodes on which some isotonic gel is applied (see Materials for instructions).

EDA100D
The EDA100D smart amplifier is used with the BN-EDA-LEAD2 leads. Usually, skin conductance is measured with dry disposable EDA electrodes on which some isotonic gel is applied (see Materials for instructions).

VU-AMS
The VU-AMS comes with its own electrode lead bundles, including one to measure skin conductance. Usually, skin conductance is measured with dry disposable EDA electrodes on which some isotonic gel is applied (see Materials for instructions).

Materials

For SSH researchers, supplies can be picked up at SOLO 1B11 in PdlC or 2.5.01 in Sylvius, see lab supplies for more info.

• Skin conductance equipment (see Setup)
• 2 dry disposable EDA electrodes
• Isotonic gel GEL101A
• Gloves
• Incidin OxyWipe
• Tissues

Electrode preparation

Since summer of 2023 Biopac EDA electrodes are not pre-gelled anymore. Instead, you have to apply the gel yourself. The electrodes can be prepared before the participant arrives.

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 GEL 101A 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 nozzle.
• 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.
• Clean the nozzle with a tissue.

The above procedure is shown in this Biopac video

​​​​​​Skin preparation

No skin preparation is required when measuring skin conductance, as it will influence the impedance of the skin and therefore the skin conductance measurement. 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.

Electrode placement

In short:

• Put on disposable gloves.
• Place two disposable EDA electrodes on the skin (typically in the middle of the non-dominant index and ring finger).
• When using PPGED-R module, attach the strap with the transmitter to the participant's wrist.
• Attach the leads to the electrodes.

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. Typically, the electrodes are placed on the inside of the fingers of the non-dominant hand (see figure). The electrodes are usually attached to the middle phalanges, as these areas have a good responsiveness and are usually not 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 PPGED-R 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 whose hands are too small), 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. For more information about possible sites for measuring electrodermal response, see van Dooren, de Vries, and Janssen (2012).

Calibration

It may be necessary to calibrate the skin conductance equipment before carrying out measurements. How this is done will differ according to the type of equipment, always check how this should be done. When using BIOPAC AcqKnowledge, make sure to enable the calibration on the start of data acquisition (see Creating an AcqKnowledge template - Add modules).

Cleaning up

• Put on disposable gloves.
• Remove the leads and, if present, the strap with the PPGED-R transmitter.
• The participant can then remove the electrodes him/herself and wash their hands.
• The disposable electrodes can be discarded in the bin.
• If necessary, the leads can be cleaned carefully with Incidin OxyWipe. 
• When a strap has been used for the wireless PPGED-R transmitter, the strap can be washed if necessary (use multiple straps to allow for drying), please contact labsupport@fsw.leidenuniv.nl when straps need to be washed.

Tips & Tricks

• Check that the electrodes are gelled properly according to the above procedure (see Electrode preparation).
• 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