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A: No alcohol should be used as it dries out the skin. If the participant must wash their hands, use just plain water. The disposable electrodes (EL507 and EL509 for MRI) are already pre-gelled, but if they are dry, GEL101A isotonic gel must be added. When using reusable electrodes, such as the TSD203, fill the cavity with GEL101A.
A: With both disposable and reusable electrodes, the surface area remains quite consistent between participants. For example, the glue of the EL507 and EL509 electrodes is quite strong so the gel cannot really spread beyond the contact area of the electrode once you apply it to the skin and the sponge pad helps to keep the gel in place.
A: If the electrodes are fresh, you do not have to do anything. If they have dried out (you will know it, as they will feel dry to the touch and may even have a crust) then add gel as per the procedure that was described during the webinar.
A: There is a downside. Since you are mixing the fresh gel with the dried gel, you are altering the salinity. Thus, the resulting gel mix will be more likely to saturate the sweat glands. Placing extra gel on the electrodes is a temporary solution until you can obtain new fresh electrodes.
A: If you are using the TSD203 EDA transducer or SS3LA reusable electrodes, you want to avoid over tightening the strap because you may occlude the vessels in the finger. The Velcro strap should be just tight enough to hold the electrode in place and prevent it from moving during the recording session. If you are using disposable electrodes EL507, you can also use surgical tape to ensure that the electrodes remain attached to the participant but the tape should not be applied too tightly, just tight enough to hold the electrode in place.
A: Subjects with cold hands will not normally provide good EDA data. Room temperature should be ambient between 22-240C—not so cold as to have the subjects feel chilled, not so hot as to have the subjects sweat. Either of these conditions will adversely impact the quality of data.
A: Yes, we have observed that cold hands will negatively impact the quality of EDA data, reducing the SCR size in some participants. This subject is also discussed in detail in the Handbook of Psychophysiology.
A: The SS3L EDA transducer contacts are filled with gel at the start of the study, then they are cleaned and stored away. Disposable electrodes are pre-gelled and if they are not stored properly or they are past the expiration date, the gel may be dry.
A: Emotional sweating across the body: Comparing 16 different skin conductance measurement locations, Physiology & Behavior 106 (2012) 298–304
A: It is best to refer to the Committee report on Guidelines for human startle eyeblink electromyographic studies.
A: The Committee report on Publication recommendations for electrodermal measurements, just like the Handbook of Physiology, refers to this and the volar phalanges placement as recommended. It does not compare the two, however, so I recommend reviewing the literature to see if there is an indication which placement is the best, thenar and hypothenar or volar phalanges. It is hard for us to make a recommendation when there are so many variables.
A: ECG electrodes should not be used to record EDA because the gel is not isotonic.
A: Please see the presentation for recommended and documented areas for electrode placement.
A: The main issue will be ensuring that you have good electrode contact with the skin. Make sure that you are using fresh electrodes that have plenty of gel and test the subject by asking them to take a breath and hold it. The electrodes should be on the participant for at least 5 minutes and this may take a little longer with some subjects. Poor peripheral circulation will also have an impact on the quality of the signal. The import thing is to test the subjects before starting the recording and making sure that you are getting good responses from them. I would also consult the literature to see if there are specific recommendations for your study population.
A: The instep of the foot and the palmar surface of the hand have the highest concentration of sweat glands on the body and are thus ideal locations to record from.
A: Impedance checking is not necessary for EDA. Isotonic GEL101A for EDA recording is not as conductive as regular electrode gel, such as GEL100, and values will naturally be lower. If you are asking how to improve the impedance for biopotential recordings, such as EMG, EEG, etc. then the following will help: abrade the skin using ELPAD or place the electrodes 5-10 minutes in advance. For a full list of recommendations, see Tips for Recording Good Data.
A: The subject is grounded by the EDA amplifier. This means that you do not have to add an additional ground on the other biopotential signals – ECG or EMG. However, if you would like to run an additional ground, without creating a possible ground loop, you should use one of the CBL205 adapters. The CBL205 plugs into the ground on the biopotential amplifier and then the ground electrode lead plugs into the other end of the adapter.
A: When using EDA, you are already grounded via the VIN- connection of the EDA100C/GSR100C amplifier. Thus, no other ground is necessary, not even an ECG ground. However, you can certainly use other grounds, just make sure to use a CBL205 cable at the ground lead connection for any additional grounds you may want to use.
A second ground can be very useful when recording EEG or EMG (because the quality of the ground connection at the finger with isotonic gel is not the best possible and for these sensitive signals it’s nice to have a separate ground) or in the event that the EDA lead gets disconnected through movement artifact, etc.
A: The hands are not a principal site for thermoregulatory sweating so the impact on recorded EDA should be minimal.
A: Please review the section on gel application to electrodes. We strongly recommend TSD203 (MP150) or SS3LA (MP36R) reusable electrodes or EL507 disposable electrodes (use only EL509 electrodes for recording in the MRI). These have been extensively documented in multiple papers. We also strongly recommend GEL101A, as other gels are not isotonic and will have a salt concentration which may impact data quality. Dry electrodes may be salvaged—see the presentation—but for best results we recommend using fresh electrodes.
A: No, an additional ground electrode is not required when recording EDA. The subject is automatically grounded through the EDA electrodes. If you want to use an additional ground electrode, use the CBL205 in series with the ground cable of the other amplifier. This is not typically recommended when only recording EDA. Please read this Grounding Notes.