Mar 062017

Recently during meditation at home just before bed, I have noticed that I sometimes start feeling sleepy midway through the period and start losing count of my breaths. To monitor this, I made it a practice to clench my jaw briefly whenever I lost count and had to return to “1”. The jaw clench serves as a convenient way to register an event in the EEG signal, as the Muse headband automatically records jaw clench events. In addition to recording EEG, I also recorded my breathing and synced the two signals together. The spectrogram below shows the EEG signal from the left front sensor. Superimposed are the (blue) breath and (red) jaw clench signals.

Highly rhythmical breath, alpha band signal (8.3 Hz) and no loss of breath count during the first 10 minutes.; irregular breathing, missing alpha band signal and loss of breath count (indicated by jaw clenches) during the final 10 minutes.

This same phenomenon occurred on four different dates, recorded just before 9:00 pm on February 9, 10, 11 and 15, 2017. I had been looking for correlates in the EEG signal to the experience of losing my focus on counting the breath. I was surprised to find such a dramatic change in breathing pattern whenever I began to lose count. The graphs below show the alpha power at the left back sensor (TP9) in green, the respiration signal in blue and jaw clench events in red.

To examine the EEG signal differences between sleepiness and zazen, we average the absolute power values of the five standard bands for the corresponding segments at locations lf, rf, lb and rb. Results are as follows:


First we notice that at the rear sensors lb and rb, the alpha signal is stronger during zazen compared to sleepiness. At the front sensors, the story is different: beta and gamma power is stronger during sleepiness compared to during zazen, but interestingly this only seems to be happening at on the left side, at sensor lf.

Mar 052017

A young meditation practitioner (26-year-old, female) joined the September 2016 7-day sesshin at Tahoma Monastery. She had some previous meditation experience, but this was her first time training with Shodo Harada Roshi. Her brainwaves were recorded once while not meditating, and on three occasions while doing zazen. Between September and February, she averaged 4 hrs/day of meditation practice. Recordings were made on:

Sep 7, 2016 – sitting quietly, but not meditating
Sep 7, 2016 – zazen before September osesshin (~600 hours previous meditation experience)
Sep 17, 2016 – zazen after September osesshin (~50 hours additional meditation)
Feb 22, 2017 – zazen after February osesshin (~1300 accumulated hours of meditation experience)

The following features stand out:

  1. Peaks in the alpha band, recorded at the left rear electrode (TP9) grow stronger with each subsequent zazen period. While not evident in the recording of the non-meditative condition, these peaks appear more pronounced in each successive meditation recording.
  2. The frequency of eye blinks while meditating is much less than when not meditating. In addition, there seems to be a trend of decreasing eye blink frequency with greater meditation experience.
  3. At the front sensors (FP1 and FP2) there seems to be growing power in the higher frequency (beta and gamma) bands relative to a lower frequency band (theta) with greater meditation experience.

Peaks in the alpha band become more prominent with zazen practice

First, let us examine the Power Spectral Density charts at the left back sensor (TP9) .

When subject is not meditating, there is no significant peak in the alpha band.

In the first recording, subjects were instructed to meditate for 20 minutes, then to stop meditating for 5 more minutes. When asked if she was able to NOT meditate during the final five minutes, the subject said,

“I think so. I was just trying to think really fast about anything, about things I was trying to remember, to remember things, to recall. And then the whole time I was just making stuff up.”

Small peaks are visible in the alpha band during zazen meditation (before osesshin)

At the end of the period, the subject remarks,

“That was really hard. It made me nervous.”


The next recording was made a week later, at the end of osesshin.

Alpha peaks begin to appear after the osesshin.

Investigator: Have you noticed a change in the quality of your meditation during week?

Subject: Definitely. Completely. I feel like I kind of earned how to meditate, actually. I don’t know. It’s not like I haven’t done it before. But now it’s starting to click now.

Investigator: Was there a kind of a key idea you that you used, or a technique you adjusted to do it?

Subject: Learning how to relax.

Investigator: How to relax?

Subject: Yeah. That’s really hard for me.

The final recording was made 5 months later after a second osesshin. The subject had practiced meditation about 4 hours per day in the intervening time.

Two distinctive peaks appear in the alpha band (5 months after the previous recording).

Investigator: How has this sesshin been going for you?

Subject: It’s been going up and down.

Investigator: How about this particular sitting right now?

Subject: I couldn’t really get into the breathing like I wanted. I couldn’t fully relax. Physically, sitting is still hard for me.

Investigator: So with your breathing, what were you aiming for?

Subject: Comfortable. Something more round. Sort of, that doesn’t feel so forced.

Investigator: But you weren’t finding that in is this round?

Subject: I didn’t really sink into it, but it wasn’t terrible.

Note that there are two distinct peaks two peaks in the alpha band as opposed to just one seen in most other subjects.The significance of this is unknown.

Similar results were obtained for the right back sensor (TP10) :

No alpha peak for the non-meditating condition.


Slight alpha peak during zazen meditation.


Small peaks in the alpha band during zazen (after sesshin).


Pair of clear alpha peaks after the February 2017 osesshin.

Eye blinks become less frequent with meditation practice

The Muse headband automatically records eye blinks. Eye blinks were anti-correlated with meditation–that is, the stronger a person’s meditation focus, the less frequently they blink their eyes. Below are the results for the present practitioner, subject S23:

Frequent eye blinking when not meditating.


Less frequent eye blinks when doing zazen (same recording as above).


After the September osesshin, eye blinking is substantially reduced.


After the February osesshin, eye blinking has nearly ceased.

Higher frequency bands develop greater power relative to lower frequency bands with greater meditation experience

Radar charts of relative band power for each of the four recordings suggested that higher frequency bands began to dominate lower frequency bands on successive recordings of meditation, after a week-long osesshin and then again after a second osesshin five months later.

Delta frequency appears to predominate, but this probably due to frequent eye blinking (see spectrogram below).

Higher frequencies (gamma and beta) appear at left front electrode (FP1).

Greater power of beta and gamma in frontal region compared to rear.

Beta and Gamma frequencies predominate.

Frequent blinking masquerades as a delta band signal as can be seen in the correlation between the low frequency patches (below 4 Hz) in yellow-orange of the spectrogram below on which the eye blink signal has been superimposed.

Eye blinks result in registering low-frequency (delta) oscillations so were therefore ignored in the subsequent analysis. See also the post, Sleuthing a delta wave mystery.

To get a better handle on the increase in high frequency oscillations with greater practice, we defined a new parameter which represents the ratio between a stand-in for high frequency (beta power) and a stand-in for low frequency (theta power). Values were normalized to be positive. Results are given below.

Ratio of beta to theta power when not meditating

Ratio of beta to theta power during zazen before first osesshin

Ratio of beta to theta power during zazen after the first osesshin

Ratio of beta to theta power during zazen after the second osesshin five months later

The ratio of beta to theta power seems to support the hypothesis of increasing high frequency oscillations in the frontal area with greater zazen practice.

Mar 052016

During one EEG recording session, a subject was asked to first meditate by following her breath for 7 minutes, then to start worrying about something for the next 7 minutes. She chose to worry about work. We see in the spectrogram below, around t=420 seconds (7-min), there is a distinct increase in low frequency oscillations at the delta and theta levels.

spectrogram of meditating, then worrying

A spectrogram is in effect a three-dimensional graph: time on the horizontal axis, frequency on the vertical axis and color patches to indicate the amplitude of each frequency component at successive times. Here we see greater intensity (indicated by orange and red color) in the lower frequencies after the 420-second mark.

We can also use radar charts to examine the relative power for each of the five frequency bands. the following two charts apply to the initial 7-minutes of meditating and to the next 7-minutes of worrying.

following breath



What is going on here? Why should the relative delta power be so strong when the subject is worrying? Aren’t delta waves associated with deep sleep?

Before continuing, we should examine the possible effects of certain artifacts in the EEG signal, especially those due to blinking the eyes. Motor impulses to the eye muscles produce electrical signals that are picked up by the EEG sensors, but these signals tell us nothing about activity in the cortex.


One possible explanation of the exceptionally high amplitude of low frequencies is that during the interval of worrying the subject blinked frequently while in the preceding interval of following her breath, she blinked only occasionally.

To explore this further, we made a recording in which another subject (S1) blinked deliberately at a rate of about once every two seconds for 20 seconds. Below are the spectrogram and immediately below it the time series graph of eye blinks. During this 40-second interval of interest, the subject blinked 11 times in 20 seconds.

spectrogram for eye blinks

It is quite clear that eye blinks result in low frequency oscillations that are readily visible in the corresponding spectrogram.

Could it be that our worrying subject was blinking her eyes more frequently during worrying than she was while meditating? Selecting time series plots of eye blinks for intervals of meditating (following the breath) and worrying, we see a distinct difference.

 S3-rec66-following-blink-PSD  S3-rec66-worrying-blink-PSD

This seems to be the most plausible explanation of the high levels of delta oscillations observed: our subject blinked her eyes much more often when worrying than when meditating. These signals probably originated in eye muscles rather than in neurons in the brain.