Twenty-five minute recordings
In an earlier post, I compared EEG patterns for short (3-minute) periods at the beginning of rounds during a “12-hour sesshin”, or “All Day Sit” at the Tahoma Zen Monastery. The three conditions I compared were counting the breath, thinking of tools and following the breath. In the first and third conditions, the alpha component predominated, while in the second (non-meditative) condition, some beta and gamma appeared stronger, especially in the left front area. However, the story does not end there, as can be seen by looking at recordings for the entire rounds.
Rounds of meditation lasted about 25-minutes. I made three recordings during three different rounds: “counting the breath”, “thinking of tools” and “following the breath (clear mind)”. The graphs below show the EEG signal for the delta band at the electrode in the right back position and the beta band at the left front position. Choice of these particular bands and sensors is arbitrary, just a quick way to view changes over the course of a 24-minute session.
Twenty-four-minute session of counting the breath. Data for delta frequency band at right back location (T10) and beta band at left front location (TP1).
Twenty-three minute session of thinking of tools.
Twenty-six minute session of following the breath
A couple of things are immediately apparent. First, the beta values (brown curves) are on average not constant, but increase gradually for the first several minutes (600 – 800 seconds) before they reach a stable level. Second, there are spikes in delta values (blue curves) that become more prominent in the latter part of the session. This suggests that when looking for patterns in these three sessions, there may be different results when we compare time intervals at the beginning versus later intervals after the signals have stabilized. I decided to compare the final ten minutes of each round.
The final ten minutes
The radar charts below show median relative power values in a 10-minute interval near the end of each session.
Ten minutes of counting the breath – median values
Ten minutes of thinking of tools – median values
Ten minutes of following the breath, with a relatively clear mind – median values
In all three charts, alpha is predominant at the back of the head. We see reduced alpha in the “thinking of tools” condition, which is not unexpected. But the high levels of beta and gamma for the meditative conditions of counting and following the breath are somewhat surprising. This pattern was not at all evident in recordings made during 3-minute intervals at the beginning of the round.
Let’s compare side-by-side EEG patterns for these three conditions at the beginning and end of each round.
counting breaths
| Counting breath – first 3 min | Counting breath – last 10 min |
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Thinking of tools
| Thinking of tools – first 3 min | Thinking of tools – last 10 min |
|---|---|
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Following breath
| Following breath – first 3 min | Following breath – last 10 min |
|---|---|
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In all three rounds, the alpha component decreased and the beta and gamma components increased over the course of the sitting.
Comparing the results of the previous post and here, the message is clear: when the subject is engaging in a practice for 20 minutes or so, the pattern in the early minutes can be quite different from the pattern revealed in later time intervals.