I'm getting lots of 60Hz hum (with plenty of harmonics) in my EEG measurements and I'm wondering how to minimize that. I followed the directions of bundling the electrode wires together so that the hum is common mode and will be subtracted when one of the electrodes is selected as the reference. This seems to work very well when doing the bucket test but not nearly as well when done on an actual head. With the bucket test if I spread out the electrodes so the spacing is comparable to the head diameter then I see a lot more hum.
I also noticed with the electrodes on my own head that if I touch grounded metal (such as my laptop) the hum is reduced a bit. And it is reduced even more when I separate my hand from the metal with a thin insulator such as a piece of paper. I presume this is creating a capacitive ground path that partly cancels out other capacitive coupling in the system.
Is it possible to wire the electrodes in pairs that are closely wrapped together where one is a reference that cancels out the hum of the other? If just put 2 electrodes next to each other on the head that does cancel hum pretty well but the potential difference is very small.
Both in the bucket and on a head, I'm getting offsets that should be plenty low (less than 10mV for most electrodes) so I don't think that's an issue.
Any other suggestions as to how to reduce the hum?