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.
Below are the corresponding PSD graphs for these sessions.
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.