An interesting feature of Power Spectral Density (PSD) graphs of EEG recordings is the peaks that sometimes appear in the spectrum. Below are two examples of spectra with peaks:
In both graphs, we note that power density varied across the beta band, manifesting itself as peaks. In the upper graph there is a double peak near the top of the beta band, around 28-30 Hz. In the lower graph, there is an additional peak near 17 Hz, as well as a strong peak near the boundary of the theta and alpha bands (about 8.5 Hz). Using only the absolute mean power across either the theta or alpha bands would not have done justice to the size of this peak.
While the best way to capture the significance of the peaks would be to mathematically integrate the Power Spectral Density over the width of the peak, this could be a challenging undertaking. I chose instead to do a semi-quantitative analysis using visual inspection of each PSD graph, along with a graphical key to assign a score from 0 to 3 indicating the “prominence” of the peak. A score of zero means that there is no peak, a score of 3 indicates a very strong peak and scores 1 and 2 correspond to intermediate conditions. The key was built using screenshots of PSD graphs from the sample.
Key for scoring alpha and beta peaks:
Alpha peaks consistently appeared around 7-12 Hz. The beta peaks I was interested in were near the top of the beta band 26-32 Hz. I scored each graph and plotted the values for beta (front sensors) and alpha (back sensors) vs hours of meditation experience:
While these plots suggest that the most prominent peaks (score=3) occur for practitioners with several thousand hours of meditation experience, they show that less prominent peaks can also be associated with extensive meditation practice.
This part of the investigation suggests that prominent (scored as 3) beta peaks in the front and alpha peaks in the back appear among more experienced practitioners but not among novices. On the other hand, there are also experienced meditators who do not show the prominent peaks. So we cannot say that we have found a definitive ‘signature’ of zazen meditation.