Origin of artifacts 12 Hz away from electrical line noise (50 Hz)? I made electroencephalographic measurements from people while they were sleeping. When plotting the frequencies, I see that I do not only have an expected 50 Hz (and 150 Hz and 250 Hz...) artifact (from nearby electrical devices) but also smaller artifacts to either side that are approximately 12 Hz apart (38 Hz and 62 Hz, 138 Hz and 162, ...) (examples from two participants):





Can someone tell me what might cause those artifacts?
Probably at exactly $\pm 12.5\text{ Hz}$, ie. some periodic phenomena that happen every four grid frequency cycles. Typical candidates for such things are rectifiers with little load.

At any rate, it's probably coming from your power grid, and you won't be able to get rid of them by anything short of notch filters.