With new graphing tools available in the Physiology Viewer 2.0, previous recordings can be reexamined and studied in more detail.
September 10, 2015 was the 5th day of a 7-day sesshin (meditation retreat) at Tahoma Monastery. At the end of the day, after the last round of formal meditation, I recorded my brainwaves, as I did each day of the retreat.
Below is a graph that was derived from an EEG recording of fifteen minutes of meditation. It shows signal power for beta and gamma frequency bands measured at the left front sensor.
We note that for a period of about 400 seconds near the beginning, there is a gradual and relatively uniform increase in beta and gamma power. Let us focus on a region of interest, the interval from 50-450 seconds.
We replot the graph for the region of interest and then label two sub-regions, each two minutes long, “first 2 min” and “4 min later”. The purpose of this is simply to have an earlier and a later region to compare.
Characteristics of the two intervals can also be compared by examining the two radar charts below. In these charts, absolute band power for all four sensors is included.
The signal at the left front sensor is of particular interest as it shows a significant increase in beta and gamma power. The Physiology Viewer, shown below provides new ways of examining the data. In particular, a spectrogram and a Power Spectral Density (PSD) graph are part of the suite data visualization tools.
We have chosen to identify two 120-second intervals with the names “zazen – first 2 min” and “zazen – 4 min later”. Selecting these individual intervals allows us to examine the EEG signal in more detail.
A spectrogram is a graph of frequency vs. time. Frequency is plotted on the vertical axis and time along the x-axis.
We select the Spectrogram for the left front sensor (lf) during the entire session. The result is a frequency vs. time graph where the intensity of each frequency is indicated by color. Here, yellow indicates greater intensity than blue. The associated color bar serves as a legend.
Note that during the “first 2 min” interval, there is less yellow in the frequency range from 12-50Hz (and hence, less power in the beta and gamma bands) than there is during the “4 min later” interval.
In addition, a horizontal yellow band runs through the entire session at a frequency value of about 8 Hz. This is in the alpha band. It will be readily apparent when we view these data using PSD graphs.
Note: Regarding the ‘vertical bands’ in the spectrogram, see a later post, Correlations of brain waves with respiration cycle.
Power Spectral Density (PSD) Graph
The PSD graph displays a spectrum of the signal. Frequency (Hz) is plotted on the horizontal axis and intensity of the signal (dB/Hz) on the vertical axis. The graphs below show the spectral composition of the two selected intervals with frequency bands indicated in color.
We see clearly how the beta and gamma intensity have increased over the course of a few minutes. While there are several peaks in the beta and gamma bands, it is unclear at this time whether a given peak is characteristic of an individual over a long time, as the alpha peak seems to be, or whether different individuals display commonly identifiable peaks.