[Eeglablist] FFT

[Kevin George Guise]

Greetings,

I'm having somewhat of an odd problem with my EEG setup and EEGlab.  Two weeks ago we made a recording and found a strange pattern in our frequency spectrum for every channel.  We found the typical 60Hz peak (which was attenuated, due to a notch filter), then a strong peak at 120 Hz, 180Hz, and so on, in increments of 60.  The same occurred for the next subject.  To test the system, I recorded a single channel from my APB under 3k ohms.  Again, I found the same thing (I increased the sampling frequency to 10,000Hz and found that this pattern persists at least up to 5000Hz.)  I've tried recording without the 60Hz filter, with various amplifier filter settings, with and without connected electrodes, etc.  We've done some detective work in the building and have found that the only change has been a new building heating system.  A vent does enter the recording room, but it doesn't seem that distance from the vent affects the power of these frequencies.  Has anyone ever seen some
thing like this, or have a suggestion on how to solve the problem?

Best
Kevin