Monday, December 30, 2013

Breathing Meditation - Alpha Amplitude

It turns out that my previous post on EEG and meditation was surprisingly popular.  The post even got one of my friends interested enough that he, too, wanted to see what happened to his EEG signals while he was meditating.  So, we hooked him up to one of my OpenBCI boards and took some measurements!  Here's the story of what we found together.

My second willing meditator.
(And the blue cap returns!)

Goal:  My goal with these recordings is simply to see if meditation has a measurable effect (any effect) on one's EEG signals.  I'm trying understand if a particular form of mental activity (ie, meditation) can be measured objectively.

Setup:  For this set of recordings, we decided to go the Full Monty and bring out the blue EEG cap (see photo above).  This was the same cap as used with the previous meditator.  We got the cap as part of a kit that we bought from Biopac.  Our cap has lots of electrodes.  We chose to use the eight electrode locations shown below.  Our reference electrode was towards the front of the head along the centerline (near FPz/AFz) and our driven ground (aka "bias") was attached behind the right ear (right mastoid).  We used the electrode gel that came with the kit from Biopac.  For data logging software, we used the OpenBCI GUI that was written in Processing.

Electrode locations used for these recordings.
Fp1, Fp2, C3, C4, P7, P8, O1, O2.
Reference electrode was near Fpz.
Driven ground ("bias") was the right mastoid.

Two Test Scenarios:  My meditating friend performed two sets of recordings: one while meditating and one while simply relaxing.  The non-meditating data will act as a baseline against which we compare the meditating data.  Note that the two recordings were done in back-to-back sessions without removing the electrode cap.

Test Procedure:  Both session started with an initial period with his eyes opened followed a long period with his eyes closed.  It is during this eyes-closed period where he was either meditating, or he was simply relaxing but not meditating.  When he is meditating, my friend's meditation style is breathing meditation, where he focuses solely on his breathing and on his body's response to his breathing.

Example Results:  Example data from his baseline recording session is shown in the spectrogram below.  This data is from an electrode on the back of his head (channel 7, which is at O1).  In this figure, you can see that once he closes his eyes, he exhibits a strong EEG signal around 10-12 Hz, which is in the Alpha band.  This eyes-closed Alpha rhythm is a very typical EEG pattern.  In this recording, there is also a faint signal between 20-25 Hz, which is simply a harmonic of the fundamental 10-12 Hz Alpha wave.  Overall, this Alpha-dominated signal seems to be very consistent with most other eyes-closed data that we've recorded from other individuals (including myself).

Example EEG data recorded during the baseline (ie, not meditating) session.  This is from the back of his head.  The horizontal stripe of signal energy is around 10-12 Hz, which is in the Alpha band.  Alpha waves are indeed commonly seen when one's eyes are closed.

Full Baseline Results:  The plot above shows data from just from one location on the head.  The figure below, by contrast, shows all eight channels of EEG data that we recorded.  It gives a fuller picture of what is happening during the baseline (non-meditating) recording session.  Like in the single example above, the plot below shows that many of the electrodes pick up the steady Alpha rhythm when he closes his eyes.  You can see, though, that the Alpha rhythm is much stronger in the back of the head than in the front.  Since this eyes-closed "posterior dominant rhythm" originates in the now-idled visual cortex (which is the back of the brain), the fact that the Alpha waves are strongest in the back of the head and weakest in the front is exactly what we would expect to see.

EEG data recorded while relaxing but not meditating.  Notice that the Alpha waves (the horizontal stripe of energy in each plot) are strongest towards the back of the head.  Click on the figure to enlarge.

EEG Data While Meditating:  Now we get to the good stuff.  Now we turn our attention to the data recorded while my friend was meditating.  The figure below shows the data recorded while he was meditating.  The meditating began when he closed his eyes, so I've limited my examination just to the eyes-closed data.  Clearly, the dominant feature is that horizontal stripe of energy in the 10-12 Hz band representing the eyes-closed Alpha rhythm.  This is the same kind of signal that we saw when he was not meditating.  So, to first glance, meditating does not have an obvious effect on his brain waves.  For example, it did not make the Alpha waves disappear nor did it make any new signals appear.  If there are any changes due to meditating, the changes must be subtle.

EEG Data recorded while meditating by focusing on his breathing.  Alpha waves still dominate.  Click on the figure to enlarge.

Change in Alpha Amplitude:  Comparing these two figures more closely, one change that I do see is that the intensity of the Alpha waves appears to decrease when he is meditating.  Because this change in amplitude is difficult to see quantitatively in the spectrograms, I replotted the data as basic spectrum plots, as shown below.  In these new plots, I've included just the eyes-closed data.  These new plots clearly show that the dominant EEG energy is between 10-13 Hz, which are the Alpha waves.  We see that the Alpha waves in both the baseline and meditating recordings are centered around 11.72 Hz, so meditating did not change the speed of his Alpha waves.  We do see, however, that the amplitude of these Alpha waves are smaller when meditating.  In fact, we see that the amplitude is cut nearly in half (6.1 uVrms down to 3.6 uVrms).  That's a pretty big change!  While we cannot yet be sure that change was caused by the meditation (repeated tests would be necessary to confirm a cause and effect relationship), this data is highly intriguing and begs for additional recordings.  This is cool.

Amplitude of the EEG signals recorded  when his eyes were closed during the baseline test (left figure) and during the meditation test (right figure).  As can be seen, the strongest signals are between 10-13 Hz, which are Alpha waves.  His Alpha are centered on 11.72 Hz.  You can see that the amplitude of his Alpha decreases while he is meditating.

Comparison of Alpha to the Meditator at Maker Faire:  Looking at my previous post for the meditator at Maker Faire, we saw that the previous mediator had very different brain patterns than seen above.  First, the meditator at Maker Faire showed no Alpha waves at all.  None.  While most people do exhibit Alpha when the eyes are closed, eyes-closed Alpha is not universal.  So, it is possible that the meditator at Maker Faire is simply one of those individuals who does not exhibit eyes-closed Alpha.  Or, as an alternate conjecture, perhaps the act of meditation suppresses Alpha waves.  Perhaps our highly-experienced meditator at Maker Faire completely suppressed his Alpha response, whereas the novice meditator shown above only showed moderate suppression of his Alpha response.  Again, we have insufficient data to make any real conclusions, but this is very intriguing.

Comparison of Beta Waves to Meditator at Maker Faire:  Another key finding from the meditator at Maker Faire was that his meditation seemed to generate EEG activity in the 15-20 Hz band, which are the low-end Beta frequency range.  His generation of Beta waves is in contrast to the novice meditator shown here, who showed no change in Beta activity.  Perhaps the lack of Beta activity is due to his inexperience, or perhaps it is due to a difference in the type of meditation.  As discussed in the Travis paper linked previosuly, different types of meditation are known to correlate with different EEG responses just as different types of mental activity can generate activity in different EEG frequency bands.  So, perhaps the Maker Faire meditator was performing a "focused attention" style of meditation (which is associated with increased Beta) whereas today's meditator was more of an "open awareness" style  of meditation (which is not associated with Beta).  I am not properly educated in the different styles of meditation, so I really should not comment on this further.  Perhaps it would be best to get the individual meditators themselves to describe their own meditation style relative to the criteria defined in the Travis paper.  That would probably be the best approach.

Conclusions:  With only a single pair of recordings from a single individual from a single sitting, we cannot draw any solid conclusions.  What we can say is that we happened to see a decrease in the amplitude of the alpha waves in the back of the head during meditation.  If this change is actually due to the meditation, it shows that the meditation is indeed having measurable changes on brain activity.  I have no idea whether changing the amplitude of the Alpha rhythm is a good or bad thing...I just think that it is interesting that we can measure any change at all.  I would love to be able to confirm this finding or to see it in other people.

Next Steps:  This has been a very basic analysis of the EEG data that we recorded.  For example, in quantifying the amplitude of the Alpha waves, I simply looked at each EEG channel in isolation from the others.  Sure, I noted that the Alpha were strongest in the back, but I did not look at any more subtle changes with how the different channels correlate with each other.  It is possible that the act of meditation brings different regions of the brain into concert with each other.  Or, maybe meditation does the opposite and causes different regions of the brain to become decoupled from each other.  Either way, some sort of quantitative analysis of the correlation between the different EEG channels might expose additional changes in brain patterns due to meditation.  I would find this kind of change to be very interesting.   I don't know what it would mean, but I would find it interesting.  So, I guess that I'm saying that I am not yet done with this particular set of EEG data.  I will pursue some kind of cross-channel analysis in my next post.

Until then, thanks for reading!

Follow-Up: Here is the analysis of the cross-channel coherence.  Cool!
Follow-Up: The raw data is available as part of the OpenBCI repository on GitHub


  1. I was sent this link...

    Note the talk of EEG changes due to meditation, specifically the comment " The 66-year-old’s brain produces a level of gamma waves – those linked to consciousness, attention, learning and memory – never before reported in neuroscience."

    According to Wiki, gamma waves are 25-100 Hz, with 40 Hz being typical. All my plots above only go to 30 Hz. If I continue to work with meditators, maybe I'll have to widen my plots to look for this gamma activity.

    1. I would like to see these reports being done while someone does Gamma breathing technique!

  2. Chip, nice contrasts and comparisons here.

    A couple points I noticed. When your subject was meditating, the frontals (especially) did seem to quiet down somewhat. In other words more deep blue throughout the spectrograms, less 'snowy'. Although this same trend is visible on all your points in comparing simple eyes closed to eyes closed & meditating.

    The other interesting item is the band around 23 hz. I think you are right on target in wanting to record the details of the types of meditation each person is doing. How active or passive that is, etc.

    There are some neurofeedback protocols that train for various alpha or gamma synchrony ('coherence') states. Les Fehmi has done a lot of work with this, his Open Focus meditation techniques and neurofeedback instrumentation.

    Happy New Year!

    1. Thanks for noting that the frontals quieted down. I wasn't quite sure what to make of that...especially since I'm not quite sure if the legitimate EEG activity is what quieter, or if the noise background (or twitchy muscle activity in the area) simply got quieter. I agree that it is intriguing!

      And yes, as you commented later, I believe that the 23 Hz energy are harmonics of the Alpha waves. I haven't confirmed this through any statistical signal processing test, but that's my belief.

      As for "coherence", I'm not sure what signal processing metric Les Fehmi uses, but the follow-on analysis that I discussed in my "Next Steps" will be looking at the spectral coherence between different electrodes on the meditator's head. I'll using mean-squared coherence as discussed in the Wiki entry: I'll look into what Les Fehmi was doing. Maybe it'll be similar, or may be it'll be different. Thanks for the pointer!

    2. Chip, hi. re: EEG quieting. I would say that it is a hallmark of meditation practices. By dropping thoughts and lightly placing attention on the breath or other object, overall activation levels decrease. And one's perceptions become finer and more sensitive. Less "busy thoughts" noise in the system, so material that was below the 'surface' of consciousness can arise, resolve, initiate healing.

      re: Open Focus. This is two things: (1) a meditation practice that you can do without any equipment whatsoever. Guided audios are available. What Fehmi noticed when he started doing this practice, were pronounced amounts of synchronized alpha waves across the whole brain. So this particular process / meditation facilitates this alpha synchony.

      (2) Conversely, Fehmi found that you can do simple neurofeedback brain training, to enhance the process. He places electrodes at Fz, Oz, T3, T4, and Cz. In other words, all the quadrant points on the 10-20 map. Then trains (rewards) for simple synchrony of alpha at these sites. One way this can be done rather straightforwardly (or primitively), is just to sum all the signals from the 5 electrodes and reward on that amplitude increasing. Since out of phase waves will cancel, the sum has the effect of rewarding for increased synchrony. I've done it, it works. You should be able to set this up fairly easily with what you have. The concept of neurofeedback rewards and 'thresholds' I could explain.

      re: coherence, yes this is a common measurement metric used in the normative databases, such as Neuroguide, NewMindMaps.

  3. What is your definition of "meditating"? Have you tried investigating brainwave entrainment through audio techniques. There's a decent CLI tool called sbagen that can generate any type of custom sequences and modulations, and it would be very interesting to see if any of the types of audio entrainment are actually modulating brainwave activity. I'd suggest relatively long runs (10-30 minutes) as entrainment may take some time to occur (if it does at all) and be visible in your scans.

    1. Hi David,

      I myself have no definition of meditation. The state of meditation is simply self-reported by the people who have asked for me to hook them up to my EEG system. I'm hoping to get a few more words from my meditators to describe their meditation approach.

      As for audio/visual entertainment, I do have some experience with visual entrainment (bright blinking lights) but none with auditory. Interestingly, at a New Year's party last night, I talked with a friend about auditory entrainment. Maybe I can hook him up and we can get some recordings!

  4. Chip, just saw your edits and the sentence, "In this recording, there is also a faint signal between 20-25 Hz, which is simply a harmonic of the fundamental 10-12 Hz Alpha wave."

    I've not seen this before on any of the neurofeedback spectrograms that I've done here. I wonder where it could be coming from? Do you have any ideas? Could there be some kind of cross modulation or some odd artifact of analog components in the sensor path? It's not present on the previous data you showed, is it?

    @David, yes audio or visual entrainment devices will show up in the EEG when done concurrently. But also have an after effect training as well. Here's a related paper:
    EEG responses to long-term audio–visual stimulation

    Still, in my thinking the meditation practices are reorganizing the brain at a much deeper and more permanent way. They are training for 'trait' vs. 'state'. And training certain mental attention / attitude skills such as "mindfulness of the present moment", "accepting what is", open heartedness, etc. Which I don't believe entrainment is ever going to reach.

    1. Hi William,

      Re: the 20-25 Hz signal, my first reaction is that it is just a simple harmonic of the fundamental Alpha waveform. In other domains (such as audio) harmonics are the rule, not the have no harmonics means that the time-domain waveform is a pure sine wave...which almost never happens. Nearly every real-world waveform deviates from a pure sine, so when i saw this 20-25 Hz energy, I simply thought it was a harmonic and moved on.

      Given that you are surprised to see them, I'm guessing that such harmonics are not common in EEG. So, while I cannot comment as to their source, I will now keep my eyes peeled as to whether they occur in other EEG recordings or not.

      And regarding the practice of "entrainment" vs "meditation", I concur that the outcome is likely to be very different. Sure, I have little experience here...but I do have (an attempt at) a physical understanding of what's happening in the brain. Alpha (or Beta or whatever) can have numerous causes. Just because one can induce certain brain rhythms through sensory entrainment, I have yet to be convinced that the result on mood/clarity/restfulness/whatever is the same when those same brain rhythms are achieved through more traditional means.

      I think entrainment is a cool phenomenon, but I think that it warrants skepticism if you're trying to do anything more profound than making funny squiggles on a EEG trace.

  5. Chip,

    In the articles you've shown me, it seems like alpha waves are often less important than beta, gamma, or theta waves, especially for ‘open monitoring’ meditation such as this. Can the OpenBCI system pick up those kinds of waves? Are they simply lower amplitude than the alpha waves?


    1. Hey Rob,

      OpenBCI can certainly pick up Beta waves. With the meditator that I recorded at Make Faire, it was Beta waves that we saw.

      It's possible that that we're not measuring on the correct location on the head. The EEG montage that we've been using is definitely weighted to the back of the head. Perhaps if we had some more electrodes along the front of the head (but not right above the eyes, as we're doing now), we might get a different story.


  6. Breath meditation is a simple and easy way to gain greater self-awareness, peace, and health. It's easy to learn and costs you nothing but a few minutes of your time. With breath meditation, you focus your attention on your breath, following its rhythmic movement as you inhale (breathe in air) and exhale (breathe out air).

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  8. hey, It looks very interesting ! Where did your friend learn meditation, did he follow any master?