Mar 092016
 

As discussed in a previous post, alpha waves account for the most common frequency peak for individuals relaxed and sitting quietly. It appears in most people during zazen meditation. The alpha peak, usually centered around 8-10 Hz is stronger in the region near the ears (lb and rb) than in the front (there is general agreement that alpha oscillations originate in the occipital lobe of the brain, at the back of the head. It is most easily observable when the subject’s  eyes are closed.

We have observed some other frequency peaks, generally at higher frequencies (beta and gamma) that appear occasionally in the EEG signal. Consider the following spectrograms for the signal from the rb sensor from four different individuals with a wide range of meditation experience.

spectrogram rec136-S1-rb

Subject S1 has roughly 5,000 hours of meditation experience. Here the alpha oscillation at 8 Hz is most prominent, which a second oscillation at 28 Hz is also visible.

 

spectrogram rec37-S6-rb

Subject S6 has about 13,000 hours of meditation experience. Interestingly, there is little evidence of alpha oscillations, but oscillations of about 28 Hz are clearly visible.

 

spectrogram rec67-S4-rb

Subject S4 has less experience with meditation. We see an alpha oscillation at 8 Hz, nothing at 28 Hz and a very clear oscillation at 50 Hz.

 

spectrogram rec123-S17-rb

Subject S17 has no previous experience with meditation. Here we see very little alpha, but considerable power at 28 Hz.

Below are the corresponding PSD graphs for these sessions.

PSD rec136-S1-rb

Peaks at 8.1 Hz and 28.2 Hz

 

PSD rec37-S6-rb

Peaks at 6.4 Hz, 9.4 Hz, 28.0 Hz and 50.0 Hz

 

PSD rec67-S4-rb

Peaks at 8.3 Hz, 9.5 Hz and 50.0 Hz

 

PSD rec123-S17-rb

Peaks at 28.2 Hz and 51.0 Hz

We see not only the prominent alpha peak at 8 Hz in some (but not all) recordings, but also peaks at 28 Hz and 50 Hz in some. The 28 Hz and 50 Hz peaks do not appear in all recordings and are not necessarily registered at all sensor locations. For subject S4, the 50 Hz peak was strongest in the right front sensor (rf), but for subject S6, the 50 Hz peak was strongest in the right back sensor (rb).

The 28 Hz peak is strongest at different places for different individuals. Subject S1 had the strongest peak at location rb, subject S6 at lb, and subject S17 at lf.

What is remarkable about these peaks is that they seem to have the same peak frequencies across different recording sessions and different individuals with varying meditation experience. When they do appear, they appear at 8 Hz, 28 Hz and 50 Hz.

There is nothing to suggest that the 50 Hz signal is an artifact from electrical line interference, as these recordings were done in the U.S. where the line frequency is 60 Hz. Also, it does not appear in all recordings, even recordings made on the same day. This suggests that the 50 Hz oscillation is indeed a signal from the cortex.

  One Response to “Remarkable frequency peaks”

  1. Hi, interesting data. would you mind sharing what instrument you used to make these recordings, and which brian sites you recorded? were you able to test the same subjects more than once with consistent findings? finally, what did the graphs look like at other time intervals different from the ones you showed here? Thanks in advance,
    Cheers
    Ric:)

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