Impedances

The higher the impedance–the resistance to passage of the signal between the scalp and the electrode metal–the poorer your signal will be and the more likely you are to have electromagnetic fields enter artifact into the signal. High impedances are usually the result of poor connections. If you have high impedances, redo your connections.

60 Hz is always and everywhere present in any US environment.

Think of the old analog radios that you had to tune in by turning the tuning dial. If you were dead on the center of the station you would get a stronger signal of music (back in the blessed days before talk radio) and little or no static/noise. If you were a little off the center, you’d start to get static, and the further off you were, the worse it would become until you couldn’t hear the programming any more.

The hookup is your tuning dial. If you could get absolute zero impedance and balanced impedances among all site pairs, you’d pick up your tiny little EEG signal loud and clear. The higher the impedance–that is, the greater resistance there is between the scalp and the metal of the electrode–the less effectively the EEG signal can pass and the more easily the “noise” of 60 Hz can enter the conductive paste and electrode. Any time you see 60 Hz high at one site but not another, it’s almost certainly a poor connection–poor prepping (remember you prep “long, not hard”), too much hair between the electrode and scalp or too little paste. Re-prepping and resetting the electrode will usually resolve the problem.

If you have high 60 Hz (that is, equal to or higher than the tallest other signals on the power spectrum), especially on both sides, this can indicate a problem with the power. Using an ungrounded outlet (pretty rare in the US, but very common here in Brazil) or sometimes using an ungrounded power supply for a laptop (2 prong plug instead of 3-prong) can be a problem. If using a laptop, unplugging the power supply FROM THE WALL–not just from the computer–can often result in a big drop in 60 Hz if that’s the problem.

Watching the oscilloscope is also very helpful. A good EEG signal is a thin line running along a fairly stable baseline (not “wandering” all over the screen). Most importantly, it is “organic”. The wave widths (frequencies) vary, getting wider and narrower, and the wave heights (amplitudes) also vary. When you have 60 Hz interference, you’ll notice that the wave form suddenly becomes very regular: It is very fast (you can count the number of wave tops in a division on the display and calculate the frequency: it will be 60). Most important, it is unvariable–mechanical rather than organic–with every wave the same width and roughly the same height.