The higher the impedance\u2013the resistance to passage of the signal between the scalp and the electrode metal\u2013the 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.<\/p>\n
60 Hz is always and everywhere present in any US environment.<\/p>\n
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\u2019d start to get static, and the further off you were, the worse it would become until you couldn\u2019t hear the programming any more.<\/p>\n
The hookup is your tuning dial. If you could get absolute zero impedance and balanced impedances among all site pairs, you\u2019d pick up your tiny little EEG signal loud and clear. The higher the impedance\u2013that is, the greater resistance there is between the scalp and the metal of the electrode\u2013the less effectively the EEG signal can pass and the more easily the \u201cnoise\u201d of 60 Hz can enter the conductive paste and electrode. Any time you see 60 Hz high at one site but not another, it\u2019s almost certainly a poor connection\u2013poor prepping (remember you prep \u201clong, not hard\u201d), too much hair between the electrode and scalp or too little paste. Re-prepping and resetting the electrode will usually resolve the problem.<\/p>\n
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\u2013not just from the computer\u2013can often result in a big drop in 60 Hz if that\u2019s the problem.<\/p>\n
Watching the oscilloscope is also very helpful. A good EEG signal is a thin line running along a fairly stable baseline (not \u201cwandering\u201d all over the screen). Most importantly, it is \u201corganic\u201d. The wave widths (frequencies) vary, getting wider and narrower, and the wave heights (amplitudes) also vary. When you have 60 Hz interference, you\u2019ll 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\u2013mechanical rather than organic\u2013with every wave the same width and roughly the same height.<\/p>\n","protected":false},"excerpt":{"rendered":"
Impedances The higher the impedance\u2013the resistance to passage of the signal between the scalp and the electrode metal\u2013the 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. […]<\/p>\n","protected":false},"author":112,"featured_media":0,"parent":0,"menu_order":16,"template":"","meta":{"_acf_changed":false},"topic":[789],"class_list":["post-39840","answer","type-answer","status-publish","hentry","answer_topic-making-sessions-run-better"],"acf":[],"yoast_head":"\n