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I have an Emotiv EPOC (EEG headset: 128 SPS, notch filter @60 Hz, felt saline contacts) I've been playing around with. Over the course of two or three hours using CCA to plot an SNR contour against frequency and time over O1, O2, P7, and P8, I noticed that a strong background signal in the alpha-wave range was existent at ~10 Hz, but then after a while of doing other things and coming back to it the strong background signal at 10 Hz disappeared but a strong background signal at a smidgen over 8 Hz was present instead.

Naturally, this leads me to ask the question: what the heck is going on? Unfortunately, that doesn't make for a good stackexchange question title, so the replacement question:

Can alpha wave frequencies vary within a single subject over the course of a single day? If they can, by how much, and under what conditions?

(if they can't, I have to start figuring out what the heck caused those artifacts...)

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What is the impedance of the electrodes? If you are any higher than about 5-10 kOhms, you're just recording noise and/or EMG artifact from the jaw muscles, etc. –  jonsca Jan 1 at 15:15
    
(as an aside, I have never used that product, but most of the "consumer" type headsets are unreliable at best) –  jonsca Jan 1 at 15:17
    
The impedance is not listed in the product specs... I'm doubtful of it being EMG, unless I'm just bad at controlling myself insofar as constantly clenched teeth are concerned (which is possible - I'll try again); the background signal was nearly constant over a period of 8 seconds using a moving window of 2 seconds with window steps of 0.25 seconds. As for noise... the headset is a fully enclosed wireless unit that transfers data after amplification. Why would such sharp systemic noise move...? I mean, I guess if I had a cell phone doing something right next to my head... Argghhh... –  user Jan 1 at 21:17

1 Answer 1

I noticed that a strong background signal in the alpha-wave range was existent at ~10 Hz, but then after a while of doing other things and coming back to it the strong background signal at 10 Hz disappeared but a strong background signal at a smidgen over 8 Hz was present instead.

Alpha-waves (7-13 Hz) are related to a dozen of things. Here is an easy first experiment for you:

  • Sit still, stare at a wall, eyes open. Remove sources of noise such as TV, radio, etc.
  • Repeat the same, but now hold your eyes closed.
  • Record the signals in both conditions for about one minute.
  • Compare the power spectrums between the two conditions: you will see a big difference in the alpha power, especially at the occipital electrodes.
  • This is actually a standard test for resting-state analysis.

Back to the actual question: What you noticed is probably about drowsiness. When one gets sleepy, the alpha power starts to increase. You can try this by watching a boring movie.

Can alpha wave frequencies vary within a single subject over the course of a single day? If they can, by how much, and under what conditions?

Yes, because of the varying cognitive load, thoughts, sleepiness, emotions, etc. The brain signals vary a lot.

Also notice that all sorts of artefacts (e.g. movement, cardiac, eye-blinks, biting of teeth) have a dramatic effect on the measured signal. If you blink your eyes it produces a signal that has 100x higher amplitude compared to the signal that comes from your brain. You cannot really say much what causes any differences in the EEG unless you would do research-level recordings with research-level equipment.

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