Normally the measured impedances of the impedance check might be higher before stimulation (doing an impedance check), but when the experiment starts after some time they might fall down. This is normal (stimulation can itself lower impedance).
First ensure that the pins and cables are ok by running impedance check with the testboard and the flat cable + CMS&DRL connected according to the following picture. If is SS20/32 then 4 flat cables need to be connected and CMS&DRL should be in the same position as the image, in any "head".
If these procedure succeeds, then the next step is lowering the impedance:
- Clean the earlobe / mastoids with alcohol
- Check that DRL and CMS are properly connected and have enough gel.(even though they might not be used in the protocol, they are neccesary)
- Ensure battery is above 50%
- Ensure headcap is the correct size
- Make sure the electrode cables are connected according to the channels selected in NIC
- Move the hair to ensure the contact between the electrode and the scalp
- Add more Saline Solution / Gel
- Make sure the hair is clean and free of sprays, oils, cream and lotions. Shampoo the hair and rinse with clear water the evening before or the morning of the test. Avoid hair conditioner or oil;
- Set a Bipolar stimulation (2 electrodes), check the sum current = 0, test impedance
- Try same with other channels and other electrode cables.
Have in mind that impedance:
- depends on skin type and can vary quite a bit across individuals
- can vary quite a bit across individuals because the long/thick hair
If this does not work and the high impedance channels are always the same, then try an impedance check with the testboard as the following image, by creating a new bipolar protocol selecting the "faulty" channels as stimulation.
If this test fails for a specific electrode cable and works for others, then the electrodes cable needs to be replaced.