During the period from September 5 to 28, three sesshins (meditation retreats) were held at the Tahoma Zen Monastery. the first and third sesshin (kosesshins) had somewhat shorter meditation schedules while the second retreat, or osesshin was characterized by more intensive meditation. Among about fifty participants, 15 volunteered to have their brainwaves recorded. Of these, 13 were able to serve as subjects on two occasions–once before and once near the end or after the osesshin. The calendar below shows the dates on which each subject’s EEG recording was made. The ‘X’ indicates attendance at the sesshin; the color of the box indicates whether the meditation recording was before (yellow) or after (orange) the osesshin.
The protocol for the study had subjects sit quietly without meditating for 5 minutes (Pre), meditate for 15 minutes (Med), again sit quietly for 5 minutes (Post) for the recordings made before the osesshin. After the osesshin, subjects simply meditated for 15 minutes (After), without Pre and Post periods of non-meditation.
Results for individual subjects are available from a separate page.
It should be noted that conducting a study like this within the sesshin schedule was quite challenging. There is precious little time between the mandatory periods of formal meditation, meals, dharma talks, exercise and work. I applaud the subjects who volunteered to participate in the study during their short rest periods after lunch and dinner.
To ascertain whether there were any differences between not meditating and meditating, or differences between before and after the osesshin, two analyses were done. The first was a semi-quantitative analysis of peaks in the Power Spectral Density graphs for each of the intervals of interest. The method used to score this data is described in more detail in a previous post, Search for a an EEG Signature for Long-time Practitioners: Part III
The bar graphs below show the average score for all subjects as per four conditions: Pre and Post (intervals of not meditating), Med and After (intervals of zazen meditation before and after osesshin). Scores for peak prominence at left and right sensors were averaged to obtain values at the front and the back.
Prominence of Peaks Analysis
Clearly, the prominence of the alpha peak at the back sensors ((TP9 and TP10) was higher during meditation (Med and After) than for sitting quietly without meditating (Pre and Post). It appears that beta peaks are also more prominent at the back during meditation compared to not meditating. It’s a little less clear what the story is at the front sensors.
The second analysis compared averages of band power for each of the standard EEG bands. The number plotted on the vertical axis is the average of the inverse logarithm (base 10) of the absolute mean power for the indicated band.
Average Power Analysis
By looking at overall power rather than the prominence of peaks, we are seeing something a little different. In the case of alpha power, we once again see that oscillations are much stronger at the back sensors than the front, which is what we had expected. But we are not seeing the difference between meditation and non-meditation that we saw in the analysis of peaks. In the case of beta power, front and back are not all that different. In comparing meditation vs. non-meditation it appears that if anything, beta power for non-meditation (Pre and Post) may actually be GREATER (though not by much) than for meditation (Med and After). The result is similar for both front and back sensors.
This seems to be a different story from what we concluded from the Prominence of Peaks analysis, suggesting that looking at peaks in the PSD spectrum may be more important than measuring overall mean power.