EEG & Entrainment

Since first realising that an electrode slapped in the middle of the forehead gives very acceptable indications of entrainment, my Pendant gets a lot more use. I have now used this setup enough, with all manner of AVS, to believe it is adequate for most AVS/entrainment purposes.

Usually I use a single electrode in the middle, just below hairline. If I’m working with an asymmetrical session, I’ll place two electrodes, one over each eye, just below the hairline. I have had more frequent “pleasing” recordings, ones that at least vaguely show something I’m hoping to see, than when I have carefully considered placement with the 10-20 system. Obviously this approach is for personal experimentation only – serious science demands repeatability, which demands standards. Herein lies one of the joys of not being a scientist – I can freely ramble on about the results of sloppy experiments and my “impressions” of what’s going on. Obviously some neurofeedback protocols also require correct placement, cases where the activity of a particular brain structure is of interest.

Anyway, I have found that forehead electrode placement frequently provides recordings that correctly correspond to my “mental state” and/or the entrainment frequency.

Although I’m mostly interested in frequency independent and broadband techniques, I still spend quite a bit of time with straight entrainment. I’ve been experimenting with means of making a simple, single frequency session more effective. For no particular reason except that it’s a nice, innocuous, general purpose relaxation frequency, I’ve been using 7.8Hz as my test frequency. That single line in MindWorkstation has undergone many permutations.

Obviously there’s been ramps from various frequencies. And steps. I’ve found 30-60 second steps more effective than ramps. Define “effective” – objective EEG recordings and subjective impressions. I’m more likely to follow the beat and end up at the target frequency if I step to it than if I ramp to it, less likely to find my thoughts drifting off into modes contrary to the entrainment.

Waveforms, entrainment types, entrainment depths and so on and so forth – as far as I can tell, the more distinct the beat, the more likely it will be followed. Playing with pulse lengths and phase shifts, it would appear that the leading edge is the significant part of the signal. Fast rise times elicit stronger entrainment over the sensory cortices, and this is reflected in frontal lobe recordings. Too fast becomes a problem – square waves tend to result in very messy recordings – many other frequencies activated in addition to target. Fast rise times also tend to sound a bit intense, which I happen to like, but others find unpleasant. Note that I’m referring to real square waves here – the things called square waves in MWS are “optimised” to avoid these problems by adding a bit of slope to the leading and trailing edges. MWS optimised square waves work beautifully, and they are particularly good for striking AudioStrobe displays.

The most interesting way I’ve found to increase EEG response, and improve my ability to “stay on task” has been to add just a little bit of frequency modulation to the signal. The following image is a screen capture from Audacity, showing a fragment of this 10 minute WAV file, 7Hz8PlusMinus0Hz5.

What we have is a signal consisting of 7.8±0.5Hz pulses, with random deviation in 0.1Hz increments. In this case, the frequency changes every 0.76923 seconds, resulting in a frequency changes approximately every 6 cycles. If you don’t believe me, measure the distance between corresponding points in cycles 5 & 6 and 6 &7 (approx 7.5 and 7.9Hz). For me it resulted in significant activity in the 7.5-8.0Hz range after less than 60 seconds of sound and light stimulation, and the activity remained consistent for the duration of the session.

Just as a by-the-way, the “fuzziness” of the waveform is the 19.2kHz AudioStrobe signal riding on the 180Hz carrier, made much more apparent in the single cycle below.

I’ll be most interested to hear whether others find these little tricks useful.

Cheers,
Craig

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