My colleagues and I have observed that some participants show no obvious electrodermal activity while being monitored. We have ruled out obvious instrument malfunction or poor contact since this seems to be a persistent characteristic of only some participants. Some of my colleagues say that they find this more common among females, although I have not observed this difference. Is there anything significant about this? Is it merely a matter of less reactive sweat glands?
I suppose you could check the impedance of the electrodes to determine if there is good contact; there could be lotion on the area of the electrode. It would be helpful if you included your preparation of the contact area. You could also try a stimulus, such as a loud noise, unpleasant odor etc. that would evoke a reaction.
I agree with Jim. Once I had a similar problem that was due to a specific medication, but this is very rare.
Sometimes if I notice extremely low or high EDA values, I ask participants to count backwards by three, starting from an high number. This usually modifies quickly their EDA response (if the instrument is working and there is no impedance).
Thanks for the answers. We usually ask participants to hold their breaths, and with most cases this reliably leads to a deflection in the EDA graph. We will try the other methods you mentioned.
No doubt the time constant of your system (including the electrode - skin interface as well as amplifiers) is not too short. Assuming there are no technical barriers to recording the response, the presence of a small fibre axonopathy would be a reason for no skin response (SSR). BW - RMS