During one EEG recording session, a subject was asked to first meditate by following her breath for 7 minutes, then to start worrying about something for the next 7 minutes. She chose to worry about work. We see in the spectrogram below, around t=420 seconds (7-min), there is a distinct increase in low frequency oscillations at the delta and theta levels.
A spectrogram is in effect a three-dimensional graph: time on the horizontal axis, frequency on the vertical axis and color patches to indicate the amplitude of each frequency component at successive times. Here we see greater intensity (indicated by orange and red color) in the lower frequencies after the 420-second mark.
We can also use radar charts to examine the relative power for each of the five frequency bands. the following two charts apply to the initial 7-minutes of meditating and to the next 7-minutes of worrying.
What is going on here? Why should the relative delta power be so strong when the subject is worrying? Aren’t delta waves associated with deep sleep?
Before continuing, we should examine the possible effects of certain artifacts in the EEG signal, especially those due to blinking the eyes. Motor impulses to the eye muscles produce electrical signals that are picked up by the EEG sensors, but these signals tell us nothing about activity in the cortex.
One possible explanation of the exceptionally high amplitude of low frequencies is that during the interval of worrying the subject blinked frequently while in the preceding interval of following her breath, she blinked only occasionally.
To explore this further, we made a recording in which another subject (S1) blinked deliberately at a rate of about once every two seconds for 20 seconds. Below are the spectrogram and immediately below it the time series graph of eye blinks. During this 40-second interval of interest, the subject blinked 11 times in 20 seconds.
It is quite clear that eye blinks result in low frequency oscillations that are readily visible in the corresponding spectrogram.
Could it be that our worrying subject was blinking her eyes more frequently during worrying than she was while meditating? Selecting time series plots of eye blinks for intervals of meditating (following the breath) and worrying, we see a distinct difference.
This seems to be the most plausible explanation of the high levels of delta oscillations observed: our subject blinked her eyes much more often when worrying than when meditating. These signals probably originated in eye muscles rather than in neurons in the brain.